The Osteology of the Reptiles/Chapter 1
CHAPTER I
THE SKULL OF REPTILES
External Appearance, Excrescences, and Chief Openings
The skull of reptiles, as of other vertebrates, has undergone many changes in adaptation to food, offensive and defensive habits. It has lost not a few bones in various forms, and others have united or formed new associations; to such an extent, indeed, that there are several in later reptiles about whose homologies there has been and yet is dispute. It has developed excrescences or horns for defense or offense, or has been covered at times with a solid armor of skin bones; but it has gained permanently no new bones, though a few have been added temporarily from the exoskeleton. The skull of carnivorous reptiles (Figs. 33, 45) is more or less elongate, like that of a wolf; insectivorous reptiles may have a more slender skull (Fig. 52 b); while those reptiles using the jaws to crush invertebrates always have a short and powerful skull (Fig. 49). The face of aquatic, fish-eating reptiles (Fig. 58) is always long, sometimes very long (Fig. 67), as in the modern gavials.
Excrescences or horns on the skull have been developed in not a few. The earliest known is that of the cotylosaurian Chilonyx, with excrescences, and the theromorph Tetraceratops, with large protuberances. Some of the later Cotylosauria, like Elginia, had horny protuberances at the back part. A few carnivorous dinosaurs have a median facial and supraorbital rugosities, as though for the support of horns or spines. In the Ceratopsia (Fig. 70 a) the development of horns and spines was carried to a remarkable degree, not only on the face but also along the posterior margin of the greatly extended skull. Perhaps of all reptiles none has surpassed some of the modern chameleons in the development of facial horns (Fig. 55 d), though not a few other lizards, like the horned lizards and moloch lizards, have many sharp protuberances and horny excrescences, which, were they magnified to the size of dinosaurs would be equally imposing. Even some turtles, like the southern Miolania, have horns upon the skull. Usually the median unpaired facial horn is borne by the nasals, as in the mammals, but in the chameleons it is formed by the maxillae. The paired facial horns are borne by the prefrontals or postorbitals. The frontals and parietals are sometimes developed into enormous crests in the dinosaurs, the supraoccipital in pterosaurs. Doubtless all such horns or protuberances were covered in life with a horny sheath.
The external nostrils (external nares) vary greatly in position. Primitively located near the extremity of the face (Figs. 2, 3, etc.), each was surrounded by the premaxilla, maxilla, nasal and lacrimal, and they almost always have the same relations with the first three of these bones, wherever located. Well separated by the premaxillae and nasals in the older reptiles, they are often confluent in later ones (Figs. 31, 32, 59, 68). They are surrounded by the maxillae in the chameleons (Fig. 55), by the nasals in the phytosaurs (Fig. 66); the nasals are often excluded from them, and the lacrimals have lost all relations with them since Permian times. In most aquatic reptiles they have receded toward the orbits, or rather the face has grown away from them, often for a long distance, as in the ichthyosaurs (Fig. 50), plesiosaurs (Fig. 46 a), proganosaurs, thalattosaurs (Fig. 61), and phytosaurs (Fig. 67). In the very slender-faced amphibious Crocodilia (Fig. 68) and Choristodera (Fig. 63), however, the nostrils retain their primitive position at the extremity of the face. They are located far back from the extremity in the volant pterodactyls (Figs. 71, 72) as in most birds.
The internal nares, or choanae, normally situated almost immediately below the external (Fig. 55), are carried back by a respiratory canal, formed by the undergrowth of the maxillae and palatines as a secondary palate, to a greater or less extent in the Cynodontia and Crocodilia (Fig. 69); in the former and in the early kinds of the latter, to the posterior border of the palatines; in the later crocodiles even into the pterygoids. A similar respiratory canal, probably separated from the cavity of the mouth by a membrane only, is characteristic of the Phytosauria (Figs. 66, 67). A partial secondary palate, formed by the union of the palatines or maxillae, with the opening only a little way back, occurs in some Chelonia and Anomodontia. In those reptiles in which the external nares are situated posteriorly, the internal nares are also (e.g., Figs. 61, 66). In the plesiosaurs only (Fig. 46), there may be a partial reversion of the respiratory canal, with the internal opening in front of the external. The internal nares, primitively (Figs. 6, 21, 47) divided by the prevomers and surrounded by the premaxillae, maxillae and palatines, may sometimes (Figs. 49, 71 c) lie between the prevomers and palatines.
The parietal or pineal foramen, very large in certain shell-eating cotylosaurs (Fig. 22), had become inconstant even in that order. It is present, so far as known, in all the Theromorpha (Figs. 33–42), and in the Therapsida (Figs. 43–45), with the exception of a few forms; in the Proganosauria, Ichthyosauria (Fig. 50), Sauropterygia (Fig. 48), the Diaptosauria (Figs. 60–62), and most lizards (Figs. 55 a, 56), but is absent in some true lizards, the chameleons, and all snakes (Fig. 59). It has been reported in certain doubtful Pseudosuchia and more or less doubtfully in a few phytosaurs and dinosaurs, but with these possible exceptions appears to be absent in all the Archosauria (Figs. 65 b, 66 b, 68, etc.) as also the Chelonia (Figs. 30, 31, 32). Usually located between the parietals anteriorly (Figs. 22, 33, 43, 44, 45, 46, etc.), it may occur between the frontals posteriorly (Fig. 55). In the Anomodontia and Gorgonopsia (Fig. 43) there is a separate bone, the preparietal, a small unpaired element of doubtful homologies, absent in other reptiles, in front of or surrounding the foramen.
The orbits, directed upward sometimes in aquatic animals (Fig. 32), but usually laterally, were primitively (Figs. 2, 3, 22, 23, 33, 43, 44, 65) surrounded by the prefrontal, postfrontal, postorbital, jugal, and lacrimal. The frontal usually forms a part of the upper margin, the maxillae sometimes below (Figs. 30, 48, 49, 55, 56, 59). In snakes (Fig. 59), only the prefrontal and postorbital may be left. Sometimes the postorbital bar is incomplete in lizards (Fig. 56), snakes (Fig. 59), and therapsids (Fig. 45 d). The prefrontal is excluded in some dinosaurs, the postorbital in Araeoscelis (Fig. 52) and Hyperodapedon (Fig. 62 d), leaving not a single element invariably associated with the orbit. Antorbital or preorbital vacuities are very characteristic of the Archosauria, occurring in all phytosaurs (Figs. 66 b, 67 a) and true pseudosuchians (Fig. 65 b, d), most Saurischia (Fig. 70 a, b) and Pterosauria (Fig. 71) and some Crocodilia. Usually there is but one, but there may be two or even three on each side in certain Theropoda (Fig. 70 a).
Openings through the skull roof,[1] back of the orbits, are characteristic of all reptiles save the Cotylosauria (Figs. 2, 4, 19, 22) and Chelonia. The upper opening, the supratemporal, arose primitively by the separation of the postorbito-squamosal bar (Fig. 33 a) from the parietal (Fig. 53 c). The lower or lateral temporal opening appeared primitively (Figs. 33, 53 a) between the squamosal and the jugal. It is bounded above by the postorbito-squamosal arch, below by the jugal, to which was added, in some of the double-arched forms, the quadratojugal (Figs. 62, 64, 65, 70 a, b). Either the upper or the lower opening may occur independently, or both together. In the Cynodontia (Fig. 45) and some other Theriodontia, with a lower temporal opening (Figs. 44 f, 45 d), the squamosal may fail to meet the postorbital above the opening, permitting the parietal to form the upper boundary in part; and this is the condition in mammals. In not a few of the Therapsida, the Dinocephalia especially (Fig. 44 b), the jugal is excluded from the lower margin by the union of the squamosal and postorbital.
All known forms of the Sauropterygia (Figs. 46, 47, 48) and Placodontia (Fig. 49) have the single opening bounded below by the squamosal and postorbital, above by the sides of the parietal, that is, it is like the upper one of those reptiles with two temporal openings. It is usually considered to be what it really appears to be, the upper temporal opening only; and its certain nature will not be determined until more is known of their terrestrial antecedents.
The intertemporal vacuity. The single temporal opening of the Squamata (Figs. 55, 54), when complete, the Ichthyosauria (Fig. 50), and certain other forms here grouped under the Parapsida, lies between the postorbito-squamosal arch and the parietal, but has, in most if not all, an additional bone helping to form its posterior or outer border, the supratemporal or tabular, for there is doubt as to its real homology. (See pages 61–69 below.)
The post-temporal opening is situated on the occipital aspect of the skull, a vacuity between the parietal, or parietal and squamosal, and the paroccipital on each side. It is present in some Cotylosauria (Fig. 21 b), and Theromorpha (Fig. 45 g), and is generally present in later reptiles though absent or vestigial in the Crocodilia.
The cranial region thus exposed by these various openings has been exposed to a greater or less degree in most Chelonia (Figs. 30–32) in a different way: by the emargination of the roof bones from behind or from behind and below, until, in some forms like the terepenes, the whole temporal roof is lost.
Posterior palatine or suborbital openings occur in most reptiles (Figs. 55, 63, 66, 69, 72) since the Theromorpha, but are absent in some turtles. They are situated between the palatines and maxillae posteriorly, and are usually also bounded in part by the ectopterygoids. They do not occur in the Cotylosauria (Figs. 6, 21 a, 24, 29) or Theromorpha (Figs. 40 c, 42 c), though present in many Therapsida.
The Skull Elements
The primitive relations of the skull elements may be discussed seriatim, with their chief modifications in later reptiles.
Premaxillae (px). Primitively short (Figs. 2, 3, 4, 19, 22, 33, 43), articulating with maxillae, nasals and prevomers, the posterior process forming a partial division between the nasal chambers. They form the anterior boundary of the external and internal nares. Four or five teeth in each.
Elongate in the strictly aquatic reptiles (Figs. 46, 47) and in the Pterosauria. In the plesiosaurs (Figs. 46, 47), pterosaurs (Fig. 72 a), some lizards (Fig. 56), and thalattosaurs (Fig. 61), a median prolongation separates the nasals, articulating directly with the frontals, in the first group (Fig. 46) sometimes directly with the parietals, separating the frontals. Edentulous in the chameleon lizards (Fig. 55), they take no part in the boundary of the nares. They are also edentulous in the turtles (Figs. 30–32), anomodonts (Fig. 44 c), some dromasaurians, the later pterodactyls (Fig. 72), most ornithischians (Fig. 70 c, d), the chameleon lizards, and many snakes. Teeth, when present, are in a single row and rarely exceed five or six in number in each, though there are as many as twenty-three in some phytosaurs (Fig. 66) and even more in ichthyosaurs, where the dentigerous border is greatly elongated. The dentigerous part is short in the long-faced plesiosaurs (Fig. 48 c). They are often fused (Fig. 72), and sometimes united with the nasals (Fig. 54 c).
Maxillae (mx). Primitively (Figs. 2 b, 3, 4, 6, 19, 21, 22, 33, 43), articulating anteriorly with premaxillae, above with septomaxillae and lacrimals, posteriorly with the jugals, ectopterygoids, and postorbitals internally, forming the outer boundary of both external and internal nares in part.
In most reptiles since Permian times they also articulate with the nasals above (e.g., Figs. 33, 43, 44); in the crocodiles (Fig. 69 b) with each other on the palate, as also in many Anomodontia and Theriodontia. They are edentulous in the Chelonia (Figs. 30–32), later Pterosauria (Fig. 72), some Anomodontia (Fig. 44 c), Dromosauria, Ophidia, Saphaeosaurus, Ornithomimus, etc. The teeth may be in single or numerous rows.
Fig. 3. Pantylus (Cotylosauria), from side. Three fourths natural size.
Septomaxillae (sx). Small bones, the so-called turbinals of reptiles, located partly within the nasal chamber, but appearing more or less on the outer side at the back part of the external nares. (Figs. 33, 44 a, b, f, 45.) Present probably in all the earliest and most early reptiles, and in most Squamata (Fig. 59); they are absent in the Chelonia (Figs. 28–32) and Crocodilia (Figs. 68, 69). In some Dromasauria they extend back on the face to meet the lacrimals. Little can be said about them in other extinct reptiles.
Nasals (na). Articulating with premaxillae, frontals, prefrontals, and lacrimals, forming more or less of the partition between the external nares.
Except in most Cotylosauria (Figs. 2, 3, 22, 23, etc.), some Theromorpha and Therapsida, they also articulate with the maxillae on the sides. They are absent in many Chelonia (Figs. 28–32); either absent or fused with the premaxillae in the Mosasauria (Fig. 54 c); separated by the premaxillae in the plesiosaurs, pterosaurs (Fig. 72), and some lizards (Fig. 56), and probably absent in some of the former (Fig. 46). They do not enter into the formation of the nares in the Rhiptoglossa (Fig. 55 d), but surround them in the Phytosauria (Figs. 66, 67). Very large in the Ichthyosauria (Fig. 50), they also articulate with the large postfrontals. They are often fused in the midline.
Frontals (fr). Primitively (Figs. 2, 4, 22, 23, etc.) articulating with nasals, prefrontals, postfrontals, parietals, and ethmoids, often forming the middle of the upper margin of the orbits.
Always present and not varying much in their relations. In the plesiosaurs (Fig. 46), pterodactyls (Fig. 72), and some lizards, they articulate directly with the premaxillae (Fig. 56), and in some of the former are separated externally in the middle. Often fused in midline (Figs. 68, 69).
Parietals (pa). Primitively (Figs. 2, 4) articulating with frontals, postfrontals, intertemporal, supratemporals, tabulars, and interparietals; below with the supraoccipital, epipterygoids, postoptics, and proötics.
In the absence (e.g., Figs. 31 b, 33, 44, 45, etc.) of the intertemporal and supratemporals, the parietals articulate directly with the squamosals and postorbitals; in the Chelonia (Fig. 30) and Crocodilia, also directly with the pterygoids. Fused in most late reptiles (e.g., Figs. 69, 72).
Interparietals (ip). Primitively (Figs. 2 a, 4, 22) back of the parietals on the superior surface of the skull, articulating with parietals, tabulars, and supraoccipital.
In the later Cotylosauria (Fig. 9), most if not all Theromorpha (Figs. 33, 42 d), some Therapsida (Figs. 44 a, d, g), they are situated on the occipital surface and are usually unpaired. A vestige, supposed to be these bones, occurs in some Crocodilia, originally named dermosupraoccipitals. They do not help form any part of the cerebral wall. Unknown or doubtful in other reptiles.
Lacrimals (la). Primitively (Figs. 2 b, 3, 22, 23, etc.) large, extending from orbit to nares, articulating with prefrontals, nasals, septomaxillae, maxillae, and jugals.
In the latest Cotylosauria (Procolophon), most Theromorpha (Fig. 33) and Therapsida (Figs. 43, 44, 45), and all other reptiles, they are excluded from the narial margin. They are small or vestigial in the Squamata, and absent in most Chelonia and in Sphenodon (Fig. 60 a). They are of extraordinary size in some Theropoda (Fig. 70 a), articulating posteriorly with the postorbitals. It has been urged by Jaekel and Gaupp that these bones are not the homologues of the mammalian lacrimal, and should be called by another name, for which postnasal and adlacrimal have been proposed.[2]
Fig. 4. Pantylus, from above. Three fourths natural size.
Prefrontals (pr). Primitively (Figs. 2, 4, 22) at the upper anterior border of the orbits, articulating with lacrimals, nasals, frontals, and postfrontals, and by a descending process with the palatines.
Never absent, though much reduced and excluded from the orbital margin in the Theropoda (Fig. 70 a). Sometimes (Fig. 70 c) they articulate with the postorbitals or postfronto-orbitals when the postfrontals are absent as discrete bones. Below, they articulate with the prevomers in the Chelonia (Fig. 30 b), with the palatines and pterygoids in the Crocodilia (Fig. 69 d). Excluded from the frontals in the Ichthyosauria (Fig. 50). Thought by some to be the homologues of the mammahan lacrimals, and so called.
Postfrontals (pf). Primitively (Figs. 2, 4, 22) at the upper posterior border of the orbits, articulating with prefrontals, frontals, parietals, the intertemporal or supratemporals when present, and with the postorbitals.
In the Chelonia (Figs. 30 a, 31 b, c, 32 a), Crocodilia (Figs. 68, 69), many mosasaurs (Fig. 54), lizards (Fig. 56) and the snakes (Fig. 59), the Pterosauria (Figs. 71, 72), Dinosauria (Fig. 70), and many Therapsida (Figs. 44 d, 45), they are absent or fused with the postorbitals which take their place. Sometimes they (Fig. 46 c, 49 a) help form the anterior boundary of the upper temporal opening. They extend forward to the nasals in the Ichthyosauria (Fig. 50).
Fig. 5. Pantylus. Cotylosaur skull: left temporal region, from without. Three fourths natural size.
Postorbitals (po). Primitively (Figs. 2, 3, 4, 5, 22) at the upper back part of the orbits, articulating with postfrontals, jugals, and squamosals. A descending process also articulates with the maxillae or ectopterygoids.
In the absence of the intertemporal and supratemporal, the postorbitalalso articulates with the parietal (Figs.28, 30, 33, 43). In the absence of the postfrontal it takes its place, often extending forward to meet the prefrontal (Figs. 30, 45 b), or even the lacrimal (Fig. 70 a) in the Theropoda. It still retains its connection with the maxillae [sic][3] in certain Chelonia (Fig. 31 b) and most snakes (Fig. 59), but not in most other reptiles. Rarely in the lizards (Fig. 56) it does not meet the squamosal. In the Crocodilia (Fig. 69) it is large, and may articulate with frontal, parietal, jugal, quadratojugal, and squamosal. It extends far back in the Chelonia (Figs. 30, 31 a), forming a large part of the temporal roof, articulating with the quadratojugal, the latter also in the Ichthyosauria (Fig. 50). It is extensive also in some of the Dinosauria (Fig. 70), supporting the paired horns of the Ceratopsia (Fig. 70 d).
Jugals (ju). Primitively (Figs. 2 b, 3, 5, 22, 33) large, forming the under boundary of the orbits, articulating in front with lacrimals and maxillae, above with postorbitals, and, by an extensive overlapping suture, with the squamosals and quadrate jugals; on the inner side perhaps with the ectopterygoids.
Absent in some Ophidia (Fig. 59 b) and some lizards (Fig. 56). In the lizards (Fig. 55) they may not articulate with the squamosals.
Intertemporals (it). An amphibian bone known only in Seymouria (Figs. 2, 19) of the Cotylosauria, intercalated between the postfrontal, parietal, supratemporal, and postorbital.
Supratemporals (st). Primitively (Figs. 2, 4, 19) articulating at the sides of the parietals with the postfrontal and postorbital anteriorly, the tabulars behind, and the squamosals on the outer side; interrupted by the otic notch in Seymouria (Figs. 2, 19).
Absent in the more specialized Cotylosauria, probably all Theromorpha, and all other reptiles save possibly the Ichthyosauria (p. 62) and Squamata (p. 65). Generally known as the posterior bone of the arch in the Squamata (Fig. 55 a, ta). [But see tabular below.]
Tabulars (t). Primitively (Figs. 2, 4, 22) on the dorsal surface of the skull in the Cotylosauria, as in the Amphibia, at the outer side of the interparietals, articulating with the squamosals and supratemporals, with the upper end of the quadrates and the outer end of the paroccipitals, whence the name "paroccipital plates" given to them by Baur. They are known to be absent in but a single genus of Cotylosauria; are probably present in most Theromorpha (Figs. 33 b, 42 d) and Therapsida (Fig. 44 g), and some "Pseudosuchia" (Fig. 65 c). The tabular has been identified by the author as the posterior bone of the arch in the Squamata (p. 62), and Ichthyosauria (p. 62), usually and perhaps correctly called the supratemporal. It is unknown in other reptiles.
Squamosals (sq). Primitively (Figs. 2, 4) articulating with tabulars and supratemporals above, in the absence of the latter directly with the parietals (Figs. 33 a, 53 a). Below, they cover the posterior part of the temporal region, extending back of the quadrate to articulate with the pterygoids (Fig. 7), overlapping extensively the quadratojugals on the sides (Fig. 33 a), and articulating in front with jugals and postorbitals.
In later reptiles the squamosal has undergone many changes, but is always present, though sometimes vestigial in the Chelonia, Squamata, and Ichthyosauria. Only in the Cotylosauria and some Theromorpha does it articulate with the pterygoids. In later forms it articulates with the paroccipital to a limited extent, supporting the head of the quadrate. In many Therapsida (Figs. 43, 44 b), but not in the more primitive Theromorpha (Fig. 33 a), it may articulate with the postorbital below as well as above the temporal opening. Its relations with the quadratojugal are also inconstant, lost in the Crocodilia (Fig. 69 c) and Predentata (Fig. 70 c). On the other hand, it may extend forward to unite with the maxillae in some plesiosaurs (Fig. 46 b). In the Squamata (Fig. 55 a), as most recent authors identify the squamosal, it articulates with the bones usually called the supratemporal and the postorbital (rarely excluded from it) and usually with the jugal.
Quadratojugals (qj). At the outer posterior side of the temporal region (Figs. 2 b, 3, 22, 33), overlapped by the squamosals, articulating in front narrowly with the jugals, behind with the quadrates.
The quadratojugals are relatively large in the primitive skull, sometimes forming a part of the articular surface for the mandible (Fig. 21 b). In the single-arched skull the quadratojugal tends to disappear. It is probably present in all Theromorpha, but is often confined chiefly to the posterior side of the quadrate (Fig. 42 d). It is absent in most Therapsida,[4] the Sauropterygia and the Squamata. It enters into the boundary of the lower temporal opening only in the Crocodilia (Fig. 69), Phytosauria (Fig. 66 b), Pseudosuchia (Fig. 65), Pterosauria, Theropoda (Fig. 70 a), and some Predentata (Fig. 70 d), excluded in many Predentata (Fig. 70 c), as in all the other double-arched reptiles. It is very large in some Chelonia (Fig. 30 a), articulating with the postorbitals, as is also the case in the Crocodilia (Fig. 69 c).
Prevomers (pv). Paired bones on the palatal surface, articulating with the premaxillae in front, the pterygoids and palatines behind separating the internal nares; dentigerous (Figs. 6, 40 c).
Only in the Chelonia (Fig. 32 b) are the prevomers single, though sometimes fused in the Rhiptoglossa, Theropoda, and Theriodontia (Figs. 43 c, 44 e). They are edentulous in all known reptiles except the Cotylosauria (Fig. 6), some Theromorpha, perhaps, certain "Pseudosuchia," Diaptosauria (Fig. 63), and Squamata. Posteriorly in the Squamata (Figs. 55 c, 56 b) they articulate with the palatines only, as also in some Chelonia (Figs. 31 a, 32 b) and Plesiosauria (Fig. 46 b). Generally believed not to represent the unpaired vomer of the mammals.
Palatines (pl). Primitively (Figs. 6, 21) forming the posterior boundary of the internal nares, articulating with the prevomers and pterygoids on their inner sides, the maxillae on their outer, and with the descending process of the prefrontals above. More or less dentigerous.
Fig. 6. Pantylus. Cotylosaur skull: from below.
Three fourths natural size.
Teeth are still present in the Theromorpha (Figs. 40 c, 42 c) and some Therapsida, but are lost in other reptiles except the Rhynchocephalia (Figs. 62 e, 63 b) and most Squamata (Fig. 54 b). They may join in the middle in the Chelonia (Figs. 31 b, 32 b) and in the Crocodilia (Fig. 69 b) below the prevomers.
Pterygoids (pt). (Figs. 6, 7, 21, 40 c.) Bones of the posterior part of the palate, articulating with the prevomers in front, the palatines and ectopterygoids laterally, the basisphenoids on the inner sides, the quadrates and squamosals posteriorly. Dentigerous.
As stated above they do not articulate with the prevomers anteriorly in the Squamata and many Chelonia and Plesiosauria, but do articulate with the parietals in many Chelonia. Their connection with the prevomers is lost in some Cynodontia (Fig. 43 c) and Rhiptoglossa. Teeth are generally present in the Theromorpha, Rhynchocephalia, and Squamata, and in some Therapsida and "Pseudosuchia." In the early reptiles (Figs. 6, 21, 24 c, 40 c) they are more or less loosely articulated with the basipterygoid process of the basisphenoid, as in most modern reptiles, but are fixed in the Therapsida (Fig. 43 c) and not a few others. There is an interpterygoidal space between them partly filled with the parasphenoid rostrum in the early reptiles (Figs. 6, 21 a), as in the Plesiosauria (Fig. 47 f), Rhynchocephalia, most Squamata (Fig. 55 c), etc.
Fig. 7. Pantylus. Cotylosaur skull: left quadrate region, with section of mandible through condyle. Enlarged one half.
In some Therapsida (Fig. 43 c), Nothosauria (Fig. 47 e), Placodontia (Fig. 49 b), they unite along the whole middle line. There is a theory that the pterygoids are the real homologues of the mammalian alisphenoids.[5]
Ectopterygoids (ec). The ectopterygoids (transpalatines) have not yet been certainly demonstrated in the early Cotylosauria, though perhaps present; they are certainly absent in some of the Temnospondyli. They have been recognized in all other orders except the Ichthyosauria and Chelonia, connecting the pterygoids with the posterior end of the maxillae, back of the palatines; sometimes also with the jugals in the Squamata (Fig. 55 c). Most remarkable are their relations in Pteranodon (Fig. 72 c) of the Pterosauria, where they pass above the palatines to unite with the pterygoid. The ectopterygoids are believed by some to be the homologues of the pterygoid process of the alisphenoid of the mammals.
Epipterygoids (ep). A pair of bones articulating below with the pterygoids, above with the parietals (Fig. 8). They have been observed in the Temnospondyli, various genera of the Cotylosauria, Theromorpha, and Therapsida, and are probably generally present in reptiles, though absent in the Crocodilia, many Chelonia, the Ophidia, Amphisbaena, Rhiptoglossa of the Squamata. In the Chelonia (Fig. 30 b) they have been identified with a plate of bone intercalated between the descending plate of the parietal and the basisphenoid of many forms. Their real homologues are yet doubtful; by some they have been identified with the alisphenoid of the mammals.
Fig. 8. Labidosaurus hamatus Cope. Cotylosaur skull: A, right quadrate from below; B, the same from above; C, posterior basicranial bones from above.
Fig. 9. Labidosaurus hamatus Cope. Cotylosaur skull: from behind.
Supraoccipital (so). (Figs. 9, 21 b, 42 d.) Unpaired, articulating with the parietals and interparietals above, the exoccipitals, paroccipitals, and proötics, and including a part of the semicircular canals formerly believed to be in a separate bone called the epiotic, but which has never been demonstrated in any air-breathing animal. Primitively more or less excluded from the margin of the foramen magnum.
Only in certain plesiosaurs is the supraoccipital paired, by the extension of the large foramen magnum to the parietal roof. In most reptiles save the Ophidia and Crocodilia, it enters more or less into the boundary of the foramen magnum.
Exoccipitals (eo). Primitively (Figs. 21 b, 42 d) small, forming the larger part of the boundary of the foramen magnum, approximated to each other both above and below, closely articulated with the basioccipital only.
Primitively the exoccipitals took but little part in the formation of the occipital condyle, but in many later reptiles they form a large part, as in the Chelonia (Fig. 31 b), or even the whole, as in the Amphisbaenia (Fig. 56 b); or, by the recession of the basioccipital, the double condyles of the Cynodontia and mammals.
Paroccipitals (po). (Figs. 9, 21 b.) Only in the Cotylosauria primitively do the paroccipitals exist as distinct bones in the adult, articulating with the exoccipitals, supraoccipital, proötics, stapes, tabulars, and quadrates. On the inner side they help form, with the supraoccipital and proötics, the otic capsule. In the Theromorpha, so far as known, the paroccipitals are fused with the supraoccipital, suturally or loosely articulated with the exoccipitals. In the Chelonia (Fig. 31 b, op), only of modern reptiles, are they separate bones in the adult, intercalated between the exoccipitals, supraoccipital, proötics, squamosal, and supporting the head of the quadrate. Among other reptiles they are known to be free only in the Ichthyosauria (Fig. 51), articulating with the basioccipital, exoccipitals, stapes, and so-called supratemporal. In other reptiles they are indistinguishably fused with the exoccipitals in the adult.
Proötics (pc). The proötics (Figs. 8, 10, 11, 30, 59, 69) are a conspicuous part of the brain-case, intercalated between the supraoccipital, paroccipitals, basioccipital, basisphenoid, and, when present, the postoptics, and containing a part of the internal organ of hearing. Their relations are yet poorly known in the primitive reptiles. They usually have foramina perforating them for the passage of the third and sixth nerves, and form the posterior boundary of the foramen for the fifth nerve; posteriorly for the eighth, ninth, and tenth nerves. They form a large part of the brain-case exteriorly in the snakes (Fig. 59) and amphisbaenian lizards; in the mosasaurs (Fig. 57) their outer extremity extends to the outer extremity of the paroccipital, articulating with the so-called supratemporals, or, as the author believes, the tabulars.
Fig. 10. A, B, Edaphosaurus. Theromorph skull: occipital complex, from without and within, natural size; C, genus indet. basisphenoid, from below; D, the same from above; E, the same from side, natural size. F, G, H, Eryops. Temnospondyl skull: basicranial bones, longitudinal and transverse sections; so, supraoccipital; po, paroccipital; eo, exoccipital; bo, basioccipital; pc, proötic; bs, basisphenoid; ps, parasphenoid; st, stapes; ep, epipterygoid.
Postoptics (as, al), (laterosphenoids, otosphenoids, "alisphenoids"). Variable and yet doubtful bones in the reptiles, apparently not homologous with the mammalian alisphenoid, though usually so called. Imperfectly known in the early reptiles, they have been recognized in the Temnospondyli, Cotylosauria, and Theromorpha, articulating as in the Crocodilia (Fig. 69) below with the basisphenoid, above with the parietal, back of the optic foramen, whence the name postoptics given to them by Cope. Between them and the proötics is the foramen for the fifth nerve. They form the lateral brain-case in the Crocodilia, but are absent in the Chelonia and snakes. In the lizards they are imperfectly ossified, and are usually lost in prepared specimens. They are present in the Rhynchocephalia and most other reptiles.
Fig. 11. A, B, C, D, Dimetrodon. Pelycosaur skull: occipital complex: A, from below; B, the same from above; C, obliquely from behind; D, the same from in front. Temnospondyl skulls: E, Eryops, section through parasphenoidal rostrum, near front part of orbit; F, Eryops, section through parasphenoidal rostrum near base; G, Cacops, section through parasphenoidal rostrum at middle of orbit.
Basisphenoid (bs). (Figs. 10, 11 d, 30.) Forming the floor of the brain-case in front of the basioccipital, continuous with the parasphenoid in front (Fig. 12),
Fig. 12. Pantylus. Cotylosaur skull and mandible: immature animal, from below. Natural size.
which is closely fused with its under side, articulating in front above with the postoptics (Fig. 69 d), behind above with the proötics (Figs. 30, 69 d), externally above with the stapes (Fig. 11 c), and externally below with the pterygoids. It lodges in front the fossa or depression for the pituitary body (Fig. 69 d).
Stapes (st). (Fig. 11 c, d.) The stapes in all early reptiles is a large bone, articulating over the auditory opening, or foramen ovale, between the paroccipital and basisphenoid, and extending toward or touching the quadrate.
Fig. 13. Pantylus. Cotylosaur skull: internal cranial bones. Natural size. A, basioccipital; B, basisphenoid; C, section at front of basisphenoid; D, section of rhinencephalic chamber opposite hind margin of orbits; E, outline of same at extreme front end of parietals; F, same in front of orbits; pal, prevomer.It is perforated near its proximal end by the foramen for a perforating artery. It is also large in the Ichthyosauria and Plesiosauria, but in most other reptiles is slender, without a perforating foramen. It is stout and short in the Amphisbaenia (Fig. 56).
Parasphenoid (ps). (Figs. 6, 10 c, d, 21 a.) A membranous, unpaired bone, firmly fused in the adult with the under side of the basisphenoid, and never a separate bone.[6] It extends far forward as a narrow rostrum in the temnospondyls (Fig. 11 e–g) and some cotylosaurs (Figs. 6, 21 a) quite to the prevomers, forming the floor to the ethmoidal cavity. This seems to be the rule in the early reptiles, though in some (Labidosaurus sp) it may not extend in front of the basisphenoid. It has been homologized by Broom with the median vomer of mammals, whence the name prevomers for the paired bones in front, the so-called vomers.
Ethmoid (eth) or Sphenethmoid. In the temnospondyl amphibians (Fig. 11 e–g), between the orbits and in front of the optic foramina, there is a pair of thin bones lying closely below the frontals and united with the parasphenoid below, enclosing an undivided cavity for the olfactory lobes, opening into the nasal and paranasal cavities in front of the orbits. Similar bones have been observed in various cotylosaurs (Fig. 13) and theromorphs, and are probably constant among early reptiles. They have been called sphenethmoids, though they have no immediate relation to the sphenoid. Probably the median ethmoidal plate arose from the fusion of these bones. There was no median ossified interorbital septum in these reptiles, and not probably any median septum. A cartilaginous interorbital septum is present in most modern reptiles but is ossified in none.
Skull Elements — Synonyms
| Ectopterygoid | = | Transpalatine, Transverse. |
| Interparietal | = | Dermosupraoccipital, Dermoöccipital, Postparietal. |
| Postoptic | = | Laterosphenoid, Otosphenoid, "Alisphenoid." |
| Paroccipital | = | Opisthotic. |
| Proötic | = | Petrosal. |
| Prearticular | = | Goniale. |
| Postsplenial | = | Preangular. |
| Splenial | = | Opercular. |
| Supratemporal | = | Supramastoid, Suprasquamosal. |
| Squamosal | = | Prosquamosal. |
| Tabular | = | "Epiotic," Postparietal [sic][7]. |
The Mandible
The mandible of reptiles was composed primitively of eight, possibly nine, separate bones, differing from that of their temnospondyl antecedents (Figs. 14, 15) only in the loss of one or two slender bones along the inner margin of the teeth, the precoronoid and intercoronoid. All of these, except the postsplenial, known in a single cotylosaur (Fig. 18), have persisted to modern times, though never all in the same reptile, none having more than six, and some but five.
The relations of these bones will be seen in the accompanying figures (16–18) and do not require a detailed description. The dentary (d) is always present and dentigerous, except in all Chelonia (Fig. 31 b, e), some Anomodontia (Fig. 44 c) and Dromasauria, some Theropoda, and the late Pterosauria (Fig. 71 e), Saphaeosaurus, etc.
Fig. 14. Trimerorhachis alleni Case. Temnospondyl skull and mandible: A, right mandible, inner side; B, the same, outer side; C, D, E, sections of mandible as designated; F, skull and mandible, left side; psp, postsplenial; cor, coronoid; icor, intercoronoid; pcor, precoronoid.
The coronoid (cor), extending along the inner margin of the teeth from near the symphysis to the hind end of the dentary on the inner side, possibly composed of two bones in some of the earliest reptiles (Fig. 18), has been restricted to a place at the posterior end of the dentary in later forms (e.g., Fig. 55 b), and may in some cases be absent. In the Plesiosauria (Fig. 25 a) it still retains its ancient character, even entering into the symphysis in some cases. In the Dinosauria, or some of them at least, it also extends far forward, or there may perhaps be a distinct bone in front, the intercoronoid or precoronoid. Primitively (Fig. 18) it often bore teeth, as in many temnospondyls, but no such teeth are known in later reptiles.
The surangular (sa), always present in reptiles, though sometimes indistinguishably fused with the articular, forms the upper margin of the mandible back of the coronoid, and the outer margin of the Meckelian orifice (Figs. 15–18). In some it may take part in the articular surface for the quadrate.
Fig. 15. Trimerorhachis insignis. Temnospondyl mandible: A, right ramus from below; B, the same from above.
The angular (an), on the inferior border posteriorly, articulating with the dentary in front, the prearticular, articular, and surangular behind, and extending to the hind angle of the jaw, is always present (Figs. 15–18). In the crocodiles (Fig. 69 c, d) it helps form the inner border of the Meckelian orifice.
Fig. 16. A, Dimetrodon incisivus Cope. Pelycosaur: left mandible, outer side. B, Dimetrodon incisivus Cope. Pelycosaur: left mandible, inner side. C, Dimetrodon incisivus Cope. Pelycosaur: right mandible, inner side. D, Labidosaurus hamatus Cope. Cotylosaur: right mandible, inner side. E, Labidosaurus hamatus Cope. Cotylosaur: right mandible, outer side.
Fig. 17. Diadectes (Nothodon). Cotylosaur: left mandible, from within. One half natural size.
Fig. 18. Pantylus. Cotylosaur: A, right mandible from inner side; B, the same from above; C, the same from below; D, sections corresponding to letters.
The splenial (sp), entering into the Meckelian symphysis primitively (Figs. 15–18), extending back to the posterior inferior Meckelian foramen, articulating with the dentary, coronoid, prearticular, and angular, is not infrequently absent. In Pantylus (Fig. 18) only, so far as known, the postsplenial corresponds to its posterior part as in the known Stegocephalia (Fig. 15). It [the splenial] never bears teeth and is more or less inconstant, absent in Sphenodon and most Chelonia. It is a thin bone and forms the cover to the Meckelian groove, whence the name opercular often given to it. Primitively (Figs. 15–18) it formed a large part of the inferior border of the mandible anteriorly, appearing on its outer face, but in all late reptiles it is restricted to the inner side. It enters into the mandibular symphysis in most long-jawed reptiles, probably an acquired character.
The prearticular (goniale) (pa), recognized only within recent years, is a thin bone, articulating with the articular behind, the angular below, the coronoid and splenial in front, forming the hind border of the posterior inferior foramen and the lower margin of the Meckelian orifice. It was present in all early reptiles (Figs. 16–18), and remains a separate bone in the modern turtles (Fig. 31 e) and young Sphenodon (Fig. 60). It was present in the Dinosaurs, Plesiosauria (Fig. 25 a), where it was first named, Ichthyosauria, and doubtless many other extinct reptiles. It is fused with the articular in the Squamata (Fig. 55 b), extending far forward in the mosasaurs (Fig. 58), ensheathed by the united angular and coronoid, splenial and dentary. It is apparently wholly absent in the Crocodilia (Fig. 69).
The articular, the only cartilage bone of the mandible, forms the cotylus, in whole or part. Distinct in all early (Figs. 16–18) and many later reptiles, it may be indistinguishably fused with the prearticular or surangular. Believed to be the malleus of the mammalian ear.
Openings in the mandible
Aside from the large opening for the entrance of nerves and blood-vessels [and jaw muscles] at the posterior upper part of the mandible (Figs. 16–18), there are in the early reptiles one or two smaller openings through the inner wall: the posterior one just in front of and below the anterior end of the large orifice, between the coronoid, angular, prearticular, and splenial, is still present in the crocodilians (Fig. 69 d). A large perforation of the outer wall of the mandible, between the angular, surangular, and dentary, is very characteristic of most Crocodilia (Fig. 69 c), Theropoda (Fig. 70 a), Phytosauria, and Pseudosuchia (Fig. 65 a, e).
A foramen posteriorly, between the prearticular and angular, is for the passage of a track of the chorda tympani nerve.
The Skull of the Cotylosauria
(Figs. 1–9, 12, 13, 16 d, e, 17–24, 25 b, c, 26–29)
Fig. 19. Seymouria haylorensis. Cotylosaur skull: from above. One half natural size, n, nasal; l, lacrimal; pf, prefrontal; pof, postfrontal; fr, frontal; it, intertemporal; st, supratemporal; do, dermoöccipital; t, tabulare.
Fig. 20. Seymouria haylorensis. Skull and pectoral girdle: from the side. One half natural size, pm, premaxilla; m, maxilla; l, lacrimal; n, nasal; pf, prefrontal; j, jugal; po, postorbital; sq, squamosal; qj, quadratojugal; cl, clavicle; ic, interclavicle; sc, scapula; c, coracoid.
But few modifications of the primitive characters of the skull are known in this order. The parietal foramen is absent in one or two genera, the supratemporals and tabulars in a few others. Teeth may occur on the coronoids. The postsplenial occurs in but one known genus [Pantylus], as also the intertemporal [Seymouria]. The interparietals are reduced and posteriorly placed in a few, and in the latest genera may be vestigial. The lacrimal in Procolophon is small, not reaching the nares. The teeth are usually conical throughout; in some genera they are obtuse and cuspidate; in the Diadectidae the posterior ones are transversely molariform; in no known forms are they sectorial. Doubtless with future discoveries other modifications of the primitive structure will be found.
Fig. 21. Cotylosaur skull: Seymouria. After Watson. A, from below. Two thirds natural size. B, occipital view. Two thirds natural size.
Fig. 22. Cotylosaur skull: Diadectes from the side and above. One half natural size.
Fig. 23. Limnoscelis paludis. Cotylosaur skull: A, from the side; B, from above, pm, premaxilla; n, nasal; l, lacrimal; m, maxilla; f, frontal; pf, prefrontal; pof, postfrontal; po, postorbital; pa, parietal; do, dermoöccipital; t, tabulare; j, jugal; sq, squamosal; qj, quadratojugal; q, quadrate; d, dentary; sur, surangular; ang, angular.
Fig. 24. Limnoscelis paludis. Cotylosaur skull: from below. Two fifths natural size, sp, splenial; pa, prearticular; st, stapes (?).
Fig. 25. A, Trinacromerum osborni Williston. Plesiosaur: left mandible to symphysis, inner side, after Williston, 1903; B, Captorhinus agtiti Cope. Cotylosaur: left mandible, from within; C, the same, outer side.
Fig. 26. Limnoscelis paludis. Cotylosaur. Outline of back of skull. Two fifths natural size.
Fig. 27. Cotylosaur. Inner side of mandibles. A, Limnoscelis paludis; B, Labidosaurus hamatus. One half natural size. art, articular; q, quadrate; sur, surangular; cor, coracoid; pa, prearticular; ang, angular; sp, splenial.
Fig. 28. Labidosaurus hamatus Cope. Cotylosaur skull: from above. Two thirds natural size.
Fig. 29. Labidosaurus hamatus Cope. Cotylosaur skull: A, from below, B, from the side. Two thirds natural size, a, articular; an, angular; bs, basisphenoid; ep, epiotic; ex, exoccipital; pp, postparietal; pt, pterygoid; q, quadrate; st, stapes.
Fig. 30. Pleurodiran skull: Macrochelys, from the side and hemisection. One half natural size.
Fig. 31. A, Pleurodiran skull: Podocnemis, from below. B, the same, from the side. One half natural size. C, Trionychoid mandible Platypeltis, from the inner side. Three halves natural size. D, Cryptodiran skull, Thalassochelys, occiput. One half natural size. E, Cryptodiran skull, Colpochelys, from above. One half natural size.
The Skull of the Chelonia
(Figs. 30–32)
Fig. 32. Trionychoid skull: Platypeltis. Natural size. A, from above;
B, from below; C, atlas of same from the side. Note abnormal proatlas.
The skull of the Testudinata or Chelonia is never elongate, though relatively slender in some of the more predaceous kinds. It always lacks the septomaxillae, postfrontals, supratemporals, dermosupraoccipitals, tabulars, ectopterygoids, ethmoids, and the parietal foramen. The nasals are usually absent, and the lacrimals are present only in some ancient forms. The prefrontals are large, meeting in the middle line. The prevomers are single and sometimes obsolete; they usually articulate above with the prefrontals. The parasphenoid has been lately recognized as a distinct bone in certain forms. There is no postoptic, but its place is taken, except in Dermochelys, by descending plates from the parietals to the pterygoids, sometimes with an intercalated epipterygoid, which, however, is usually absent. The quadrate may or may not reach the basisphenoid. The palatines often meet in the middle line between the prevomer and the pterygoids. The pterygoids also usually meet in the middle, though separated in the Trionychoidea by the basisphenoid. The palatines also often meet for a short distance below and in front of the internal nares, forming a rudimentary secondary palate.
The temporal region primitively was wholly roofed over, and yet is, in some marine turtles, by the large postorbital, quadratojugal, and squamosal. Usually it is more or less exposed by the emargination of the roof from behind or below, or from both sides; and the squamosals and quadratojugals may even become vestigial in the process as in the terapenes. The quadrate is always large, its ear-cavity sometimes wholly surrounded by bone. The stapes is slender. The condyle is largely formed by the exoccipitals, in some wholly so. It remains cartilaginous in the Dermochelyidae, as in some cotylosaurs. The paroccipital remains free throughout life.
The mandibles have a large, free prearticular, usually but incorrectly called the splenial; the splenial is rarely present (Emydura, Toxochelys, etc.). Both upper and lower jaws are encased in a cutting horny sheath, and are without teeth. Small teeth on the palatal bones are known to occur only in Stegochelys, a Triassic genus.
The Skull of the Theromorpha
(Figs. 10 a–d, 11 a–d, 16 a–c, 32–42)
More modifications of the skull structure are found in this order than in the Cotylosauria, as would be expected. The interparietals and tabulars are probably sometimes absent, and they are always confined to the occipital surface when present, the former usually, if not always, unpaired. The supratemporals are doubtfully present in any. The quadratojugal is smaller and never extends far forward.
Fig. 32 bis. Theromorph skull: Ophiacodon mirus Marsh, lateral view. pa, parietal; po, postorbital; pf, prefrontal; l, lacrimal; j, jugal; qj, quadratojugal; q, quadrate.
The lacrimals seldom extend to the nares. The teeth are often wanting on the prevomers and are sometimes present on the coronoids. There is a lower temporal opening, bounded by the jugal, postorbital, and squamosal. In the Edaphosauridae only may it possibly extend to the parietal. The teeth of jaws and mandibles are more variable, often markedly anisodont, conical, obtuse, or compressed and sectorial.
Fig. 33. Theromorph skull: Sphenacodon. A, from the side; B, from above.
One third natural size.
Fig. 34. Theromorph skull: Glaucosaurus megalops, from the side and from above. Natural size.
Fig. 35. Theromorph skull: Mycterosaurus, from the side. Natural size.
Fig. 36. Theromorph skull: Varanosaurus brevirostris Williston, from the side. Two thirds natural size.
Fig. 37. Theromorph skull: Varanosaurus brevirostris Williston, from above. Two thirds natural size.
Fig. 38. Theromorph skull: Varanosaurus acutirostris Broili, from above. Two thirds natural size. After Broili.
Fig. 39. Theromorph skull: Mycterosaurus, from above. Natural size.
Fig. 40. Theromorph skull: Naosaurus claviger. A, from the side; B, right mandible, from inner side; C, skull from above; D, the same from below. Two fifths natural size.
Fig. 41. Theromorph skull: Casea broilii Williston, from above. Natural size.
Fig. 42. Casea broilii Williston: A, skull, from the side; B, left mandible of same, from inner side; C, skull, from below; D, from behind. Three fifths natural size. pm, premaxilla; m, maxilla; po, postorbital; j, jugal; d, dentary; sa, surangular; ang, angular; sp, splenial; art, articular; pa, prearticular; c, coronoid; poc, paroccipital; pa, palatine; bs, basisphenoid; bo, basioccipital; pt, pterygoid; q, quadrate; st, stapes; eo, exoccipital; so, supraoccipital; ds, dermosupraoccipital.
The Skull of the Therapsida
(Figs. 43–45)
Many more modifications of the skull are found among the reptiles grouped under the name Therapsida or Anomodontia sens. lat. The supratemporals are never present. The postfrontals are often absent; the quadratojugals are present only in the Dinocephalia and are small.[8] Only in some of the Dromasauria do the lacrimals and septomaxillae exclude the maxillae from union with the nasals. There is a separate bone in front or surrounding the parietal foramen in the Anomodontia and Gorgonopsia. The parietals may be united in some of the Bauriasauria. The interparietal or dermosupraoccipital is always on the occipital surface of the bone and is unpaired; it is generally present, as also the tabulars. The temporal foramen, usually bounded above as in the Theromorpha, reaches the parietal in Therocephalia and Theriodontia, the postorbitals and squamosals not meeting. The vomers are fused into a single bone in the Gorgonopsia, Bauriasauria, and Cynodontia. The pterygoids and palatines meet in the middle line in the Dinocephalia. There is a partial false or secondary palate formed by the union of the maxillae in front of the nares in the Anomodontia, a well-formed secondary palate in the Bauriasauria and Cynodontia. The ectopterygoids may be absent or present. Only in some of the Therocephalia are there teeth on the palatal bones. The pterygoids do not meet the small quadrates in the Cynodontia. In the Cynodontia the condyle is essentially dicondylar. The parietal foramen is usually absent.
Some Dromasauria and the females of some Anomodontia are edentulous. Other anomodonts may have a single caniniform tooth in each jaw, or canines and molars. The Therocephalia have anisodont sectorial teeth, the Cynodontia with real heterodont dentition, the molars sectorial or cuspidate.
Fig. 43. Gorgonopsian skull: Scylacops capensis. A, from the side; B, from above; C, from below. One half natural size. After Broom.
Fig. 44. Therapsid skulls: A, B, Mormosaurus (Dinocephalia) from above and from side. After Watson. One twelfth natural size. C, Dicynodon (Anomodontia) from the side. After Broom. One half natural size. D, Cistecephalus (Anomodontia), from above. After Broom. One half natural size. E, Gorgonops (Gorgonopsia), from below. After Watson. One fourth natural size. F, Scylacosaurus (Therocephalia), from the side. After Broom. Two sevenths natural size. G, Diademodon (Cynodontia), occiput. After Watson. One half natural size.
Fig. 45. Therapsid skulls: A, Cynognathus platyceps (Cynodontia), from the side. After Broom. One third natural size. B, the same, from above. C, Cynognathus crateronotus, upper teeth, from the side. D, Bauria (Bauriasauria), from above. After Broom. One half natural size.
The Skull of the Nothosauria
(Fig. 47)
[No manuscript. See pages 211, 246, 247]
The Skull of the Plesiosauria
(Figs. 46–48, 25 a)
The extreme aquatic adaptations of the plesiosaurs have caused certain modifications in the structure and relations of the bones of the skull that are unique among reptiles.
The general shape of the skull seems to bear a definite relation to the length of the neck, always shorter in the long-necked forms, slender, sometimes very slender in the short-necked types. The premaxillae are always greatly elongated, extending back at least as far as the front part of the orbit, in the long-faced kinds even to articulate with the parietals, above or between the frontals. The alveolar border also is [short?] shut. The maxillae are much more elongate than in the icthyosaurs and phytosaurs. There are no teeth in the palate. The nasals have never been certainly determined; possibly they are fused with the frontals, which extend as far forward as the external nares, forming the inner border. The prefrontals are small; the lacrimals have been identified in a few forms only; they appear to be absent in some. The postfrontals are probably present in all. The orbits are bounded below by the jugals and maxillae. The quadratojugals are conceded to be absent in all plesiosaurs. The single large temporal opening is bounded below by the postorbitals and squamosal, above by the parietals, which are more or less elevated in the middle in a crest. There is a pineal foramen. The squamosals, in some at least, join each other back of the parietals on the upper surface of the skull. There are no interparietals, tabulars, or supratemporals.
On the occipital surface the supraoccipital is excavated more or less by the foramen magnum, which may extend to the roof, dividing the bone. The paroccipitals are always fused with the exoccipitals. The brain-case is more or less open in front on the sides, the postoptics either reduced or absent. The stapes is large and stout.
Fig. 46. Sauropterygian skulls: A, Peloneustes. After Linder. One ninth natural size. B, Plesiosaurus. After Andrews. One sixth natural size. C, Muraenosaurus. After Andrews. One sixth natural size.
Fig. 47. Sauropterygian skulls: A, Nothosaurus, from above. One fourth natural size. B, Simosaurus, from below. After Jaekel. One fourth natural size. C, Thaumatosaurus, from below. After Fraas. One fourth natural size.
Very great also, are the modifications of the palate. The anterior nares are situated either between the prevomers and the maxillae, or between them and the palatines, and, like the external ones, are small. There is always a remarkable posterior interpterygoidal vacuity, divided in the middle throughout by the large parasphenoid, the pterygoids meeting in front of and to a slight extent behind them.
Fig. 48. Plesiosaur skulls: A, Elasmosaurus, from the side: pm, premaxilla; m, maxilla; po, postorbital; j, jugal. B, Plesiosaurus, from the side. One sixth natural size. C, Trinacromerum, from the side: ang, angular; d, dentary; pm, premaxilla; po, postorbital; j, jugal; sur, surangular.
An anterior interpterygoidal vacuity, as also posterior palatine and other openings in the palate, may or may not be present. The internal nares are in front, sometimes very much in front, of the external nares. The coronoids are elongate bones, extending along the sides of the teeth internally and meeting each other in some forms in a median symphysis. As usual in long-faced forms, the splenials meet in a median symphysis.
The Skull of the Placodontia
(Fig. 49)
Fig. 49. Placodont skulls: A–D, Placodus. After Broili. One fourth natural size.
E, Placochelys. After Jaekel. One half natural size.
The skull of the Placodontia is almost unique among reptiles for the extraordinary development of large, flat, crushing teeth upon the jaws and palatines, in Placodus as few as twenty all told, in Placochelys still fewer. In consequence, the palatines are very large, meeting each other throughout as do the pterygoids in the median line. The ectopterygoids are very small and the pterygoids are restricted to the posterior part of the palate, widely separated from the prevomers.
The massive cranium has a large temporal opening bounded above by the parietal, below by the united postorbitals and squamosal, with the postfrontal entering into the anterior border. Except for the postfrontals, the structure here, it is seen, is like that of the Dinocephalia, and possibly has arisen in the same way. The stout lateral bar below the opening is identified by Jaekel in Plachochelys as composed of the squamosal and quadratojugal, by Huene as the supratemporal and squamosal; both views are probably incorrect, since Broili finds only the squamosal, which is in Placodus the more probable. So, also, Huene believes there is an interparietal, which Broili cannot find.
The nasal only of the roof bones is unpaired in Placodus; possibly the prevomers are also single. There is a large epipterygoid. No tabulars have been found. The premaxillae in Placodus are large, each with three incisor-like teeth. The largest skulls of Placodus are about ten inches long.
The Skull of the Ichthyosauria
(Figs. 50, 51)
The skull of the ichthyosaurs, while retaining not a few primitive characters, has been highly and peculiarly modified in many ways. The greatly elongated premaxilla, unlike those of other aquatic reptiles, is broadly separated above by the very large nasal, and bears numerous teeth; the maxillae are short. All bones are paired. The frontals are small. The very large orbits have the usual bounding bones, prefrontal, postfrontal, postorbital, jugal, and lacrimal, but their relations are somewhat changed. The prefrontals are long, the postfrontals are extraordinarily large, articulating in front not only with the whole extent of the frontals but also with the nasals and prefrontals, posteriorly with the so-called supratemporals. The postorbitals are long bones forming nearly the whole posterior boundary of the orbits, with their usual articulations. The jugals are long, articulating with postorbitals, quadratojugals, maxillae, and lacrimals. The relations of the bones of the palate and the boundaries of the nares are primitive; an ectopterygoid has not been recognized and is probably absent; there are no teeth on the palatal bones. On the occiput the paroccipitals, unlike all other reptiles since the primitive Cotylosauria, save the Chelonia, are separate. The stapes is a short, stout bone, possibly an acquired, more probably a primitive, character. There are no dermosupraoccipitals. The large parietal foramen is at the front end of the parietals, sometimes between the frontals.
Fig. 50. Ichthyosaur skull: Baptanodon (Ophthalmosaurus), from the side, from above, and from below. After Gilmore.
Most characteristic of the ichthyosaur skull is the structure of the temporal region, about which there has been dispute from the time of Owen to the present. The large temporal vacuity is admittedly the upper one, bounded on the inner side by the parietal, on the outer by the postfrontal and the so-called supratemporal. There is no lateral foramen, and it is quite improbable that a preëxistent one was later closed by the encroachment of the orbit. This region, as in the primitive skull, has five bones. About three, the postfrontal, postorbital, and quadratojugal, there can be no question of identity. And unless we accept the wholly improbable theory that new bones have been developed in the temporal region of the ichthyosaurs, the other two must be homologized with the supratemporal, or tabular, and the squamosal.
Fig. 51. Ichthyosaur skull: Baptanodon (Ophthalmosaurus), from the rear. After Gilmore. Ang, angular; bs, basisphenoid; d, dentary; f, frontal.
The supratemporal bone was the first to be lost in the primitive skull, and there is no certain evidence yet forthcoming that it was retained in any reptiles after the cotylosaurs. If, however, the supratemporal was persistent in the ichthyosaurs instead of the tabular, by no possibility can it be the bone on the outer side of the squamosal, as some recent writers assert, as a comparison of the cotylosaur skull will make evident. The outer bone, sometimes obsolete in ichthyosaurs, must be the squamosal. The upper, posterior bone completing the upper border of the temporal vacuity, the author prefers to believe is the tabular and not the supratemporal, and doubtless is homologous with the bone so recognized in the skull of the Squamata. We cannot conceive of its being anything else, having as it does the same relations with paroccipital, parietal, and quadrate, rarely in the mosasaurs extending forward to articulate with the postorbital.
The Skull of the Protorosauria
(Figs. 52, 53 c–e)
In the order here provisionally called the Protorosauria the skull is completely known in none, but best in Araeoscelis, the oldest certainly-known reptile with a single typical upper temporal vacuity. The roof bones are all paired in all, so far as known. In Araeoscelis, Pleurosaurus, and probably Protorosaurus, there is a parietal foramen, but none in Saphaeosaurus. The lacrimal is small or vestigial in all. The postfrontal is present in Araeoscelis, and only in this genus are there indications of the presence of the dermosupraoccipitals. Probably all have teeth on the palatal bones.
Fig. 52. Parapsid skulls: A, Pleurosaurus, from the side. Natural size. B, Araeoscelis, from the side. Twice natural size.
Their chief interest lies in the structure of the temporal region. In Araeoscelis the temporal opening is bounded externally by three bones, the postorbital in front, and two bones posteriorly, about which there is doubt because of their evident identity with the corresponding bones in the lizard skull, which have been the subject of more controversy than any others of the reptilian skull. Aside from the tabular, there are three recognized bones of the primitive temporal region, all present in the Cotylosauria and Ichthyosauria, to which the names mastoid, supramastoid, squamosal, suprasquamosal, prosquamosal, temporal, supratemporal, and quadratojugal have been applied in almost all possible combinations. Only two of these are present in Araeoscelis, Pleurosaurus and the Squamata, to which all of these names have been given by different authors.
Fig. 53. Parapsid, etc., skulls: A, B, Mycterosaurus (Theromorpha), from the side and from above. Natural size. C, Araeoscelis (Protorosauria), from above. Natural size. D, Pleurosaurus (Protorosauria), from above. One half natural size. E, Sauranodon (Protorosauria), from above. Natural size.
The more general opinion is that the posterior one of the Araeoscelis and Squamata arch is the supratemporal, the anterior and outer one the squamosal; some, however, reverse these names, calling the posterior one the supratemporal or its synonym, the prosquamosal. Yet others both in the past and the present call the outer anterior bone the quadratojugal. The author has given his reasons for believing that the posterior bone is none of these but the tabular instead, the anterior one the squamosal, the quadratojugal absent. He believes that the posterior is the tabular because it occupies the primitive position of that bone in its relations to the interparietal, paroccipital, squamosal, and quadrate. The supratemporal is the first bone to disappear in the temporal region of the Cotylosauria, and its presence has never been positively determined in the Theromorpha and Therapsida.
It is quite possible, however, that both the tabular and supratemporal have disappeared in these reptiles, and that the squamosal has usurped their position and functions; the true supratemporal, however, has no relations with the quadrate as has the bone so called in the skull of the lizards. If so, the bone articulating with it in front and forming the outer boundary of the temporal opening may be the quadratojugal, as was formerly believed and yet is by some. It is a fact, however, that the quadratojugal is a very inconstant bone in all single-arched reptiles otherwise. It is very small in the Theromorpha, is present in only a very few of the Therapsida as more or less of a vestige, and has wholly disappeared in the Sauropterygia. That it should lose its original position at the lower outer side of the quadrate, to form part of the articular surface for its upper end, seems improbable. Furthermore, in the Ichthyosauria (and ?Saphaeosaurus) there is a distinct bone between it and the temporal opening that must be either the squamosal or supratemporal. There is at present no certain solution of the problem.
The Skull of the Squamata
(Figs. 54–59)
The skull of the Squamata is at once distinguished from that of all other reptiles by the movable, streptostylic quadrate, secondarily more or less fixed in some forms. The exoccipitals and paroccipitals are always fused; the pterygoids never reach the vomers; the interparietals and either the supratemporals or tabulars, or the quadratojugals, according to the identification, are absent. The teeth are acrodont or pleurodont; the prearticular of the mandibles is always fused with the articular. Other characters are very variable in this extensive order, which is sometimes divided into two or three distinct orders.
Sauria or Lacertilia
In the lizards (Figs. 55, 56) the quadrate articulates above normally with three bones, the squamosal, paroccipital, and a third bone whose homology is yet disputed, but which is usually called the supratemporal. The squamosal may be absent in those lizards without a temporal arch, and rarely in certain degraded burrowing lizards (Fig. 56) the "supratemporal" may also be absent, the quadrate lying against the brain-case and more or less fixed by the pterygoid. The paroccipital usually but not always helps support the quadrate.
As regards the identity of all these bones, there has been great difference of opinion, and there is by no means unanimity at the present time. The tabular, as here identified, has been called the squamosal, supramastoid, supratemporal, and even the paroccipital (opisthotic). The squamosal as here considered has been called the quadratojugal, supratemporal, paraquadrate, squamosal, and prosquamosal. The reasons for their identification as the tabular and squamosal will be found in the discussion of the skull of the Protorosauria.
Below, the quadrate articulates with the pterygoid on the inner side by a rather free joint in most lizards, in some, like the Amphisbaenia (Fig. 56) by a close sutural joint. On the inner side the usually slender stapes abuts against the quadrate (Fig. 55 c). In the mosasaurs there is an elongated suprastapedial process arching backward and often extending to the lower end, enclosing the auditory meatus, as in some turtles.
The tabular (Fig. 55 a, ta), or supratemporal, at the distal and under side of the parietal process, forming more or less of the boundary of the temporal opening, articulates with the squamosal, paroccipital, and quadrate. In the mosasaurs (Fig. 54 c) only, it has a long internal process, firmly wedged in between the paroccipital and proötic, extending nearly or quite to the semicircular canals. In some lizards the tabular has suffered reduction or has become indistinguishably fused with the squamosal.
The squamosal normally articulates with the tabular and quadrate posteriorly, anteriorly with the postorbital, and often, both in the lizards and mosasaurs, by a slender prolongation with the tip of the jugal; very rarely, as in Uromastix, with the jugal only. In some lizards it has suffered reduction, and is absent in the Geckonidae, Anniella, and Amphisbaenidae, vestigial in the Helodermatidae.
Fig. 54. Mosasaur skulls: Upper figure, Clidastes, from the side; middle figure, Platecarpus, from below; lower figure, Tylosaurus, from above. an, angular; bs, basisphenoid; c, coronoid; ep, epipterygoid; fr, frontal; j, jugal; l, lacrimal; m, maxilla; na, nasal; oc, occipital condyle; pa, parietal, palatine; pm, premaxilla; pf, prefrontal; pt, pterygoid; po, postorbital; q, quadrate; sp, splenial; sq, squamosal; tr, transverse; v, vomer.
Fig. 55. Lacertilian skulls: A, Conolophus, from above. Natural size. B, the same, left mandible. Three fourths natural size. C, Varanus, from below. Natural size. D, E, F, Chameleon, from the side, the postoptic, and the upper end of quadrate.
Fig. 56. Lacertilian skull: A, B, C, Amphisbaena, from above, below, and the side. Five halves natural size. D, side view of atlas and axis.
The temporal fossa, normally (Fig. 55 a) bounded above by the parietal, below by the tabular, squamosal, and postorbital, may be wholly absent, as in the Amphisbaenidae (Fig. 56 a), completely roofed over by dermal bones, or obliterated by the union of the temporal arch with the parietal.
The premaxillae may be paired or fused; in the mosasaurs (Fig. 54 c) the united bone is fused with the nasals posteriorly, or the latter may be absent or vestigial. The lacrimals are always small, sometimes vestigial or absent. The prefrontals are always large, entering into the formation of the nares in the Varanidae and Mosasauridae (Fig. 54 a). They articulate with the palatine by a descending process. The nasals, usually paired, are sometimes fused with the premaxillae or with each other; they are separated from the nares in the Rhiptoglossa (Fig. 55 d). The postfrontals are rarely large in lizards and are often absent; when absent the postorbitals take their place, sometimes (Fig. 55 d) ending forward over the orbit to meet the prefrontal. The postfrontal and postorbital are not rarely found united by suture in the mosasaurs; usually, however, the two bones are indistinguishably fused or the postfrontal is absent. Posteriorly the postorbitals articulate as usual with the squamosal; below with the jugals. The postorbito-jugal and the postorbito-squamosal arch may be absent in various terrestrial lizards.
The jugal is a slender bone bordering the orbit below and extending forward to meet the lacrimal when that bone is present. It articulates with the maxilla, ectopterygoid, postorbital, and often with the tip of the squamosal. It may be vestigial or even entirely absent in lizards.
The maxilla articulates normally with the premaxilla, sometimes with the nasal and prefrontal, with the jugal, prevomer, palatine, ectopterygoid. It always bears a single row of acrodont or pleurodont, pointed or obtuse teeth.
On the palate the prevomers are paired or partially fused in the Rhiptoglossa. They articulate in front (Fig. 55 c) with the premaxillae, laterally usually with the maxillae, posteriorly with the palatines only. They very rarely bear small teeth.
The palatines (Fig. 55 c), unlike those of most other reptiles, are intercalated between the prevomers and pterygoids, articulating on the sides with the maxillae and more or less with the ectopterygoids. They sometimes bear teeth. The pterygoids have the normal articulations except that in front they articulate with the palatines only. The posterior palatine opening is usually large. They usually bear teeth.
The epipterygoid, a slender rod, is present so far as known in all lizards except the Amphisbaenia and Rhiptoglossa,[9] articulating in a pit on the upper side of the pterygoids and extending to or toward the parietals.
The frontals usually and the parietals always are fused in the midline (Fig. 55 a).[10]
Fig. 57. Platecarpus, occipital view. ba, basioccipital; eo, exoccipital; pf, postfrontal; st, stapes; pt, pterygoid; q, quadrate.
The parietal foramen, usually present, is absent in many terrestrial lizards and in the Rhiptoglossa. The frontals and parietals may be either paired or unpaired. The frontals in the Varanidae, Helodermatidae, and some others have descending processes of the frontals which meet in the middle below, enclosing a rhinencephalic chamber, very much like the primitive one of the early reptiles.
The brain-case of lizards, as of other reptiles, is formed by the supraoccipital, exoccipitals, paroccipitals, basioccipital, basisphenoid, proötics, and postoptics, but is more or less membranous in front on the sides. The postoptics (Fig. 55 d, al) are small ossifications in the wall membrane, usually lost in maceration. In the Amphisbaenia (Fig. 56 c) and Mosasauria the sides of the parietals are partially decurved, forming incomplete cerebral walls, but they do not reach, as in the snakes (Fig. 59 b), to the basisphenoid.
Fig. 58. Mosasaur mandible: Clidastes, inner side of right mandible. ang, angular; art, articular; cor, coronoid; pa, prearticular; sur, surangular.
The mandibles (Figs. 55 b, 58) are composed of the dentary coronoid, surangular, articular, angular, and splenial, with a long fused prearticular, which in the mosasaurs is more or less ensheathed by the union of the coronoid and angular, strengthening the peculiar joint between the angular and splenial; a similar joint, though less well developed, is found in the monitor lizards. The mandibles are usually united in front by suture but are ligamentously connected in the mosasaurs and some land lizards.
As is seen, there are many variations in the skull of the lizards, more than in many other groups of reptiles called orders.
Ophidia or Serpentes
(Figs. 59 a–e)
The skull of snakes differs from that of lizards, especially in the complete closure of the brain cavity in front by descending plates from the parietals and frontals, the former always meeting the basisphenoid below, the latter sometimes interrupted by the coalesced optic foramina; in the constant absence of the postoptics, epipterygoids, and squamosals, the quadrate articulates proximally with the tabular only, which may also be absent. The parietals are always fused; there is no parietal foramen. There is no temporal arch, and, rarely, no ectopterygoid. The premaxillae are small and often edentulous, the maxillae rarely edentulous. The pterygoids and palatines usually bear long teeth. The postorbitals may meet the maxillae below, and there is no jugal.
The vipers (Fig. 59 e) have but one functional tooth attached to the maxilla. It is hollow, with an opening at its base and another near its apex for the passage of venom. Only the dentary is freely articulated in the mandible, the posterior bones closely fused; the two mandibles are usually united in front by ligament only. There is no ossified interorbital septum, and the proötics are largely exposed on the side of the skull.
The mandible of Ophidia has the primitive structure, except that the coronoid appears to be absent or fused, the bone usually so called being clearly the prearticular. The long splenial, as usual in reptiles with a long median symphysis of the mandibles, enters into the symphysis.
The conical teeth of the premaxillae, maxillae, and dentaries primitively were inserted in sockets, but in the more specialized types are rather loosely lodged in grooves.
Fig. 59. Ophidian skulls: A, B, C, D, Python, from above, and from the side, occiput, and palatine bone with teeth. Natural size. E, Crotalus, from the side. Natural size.
The Skull of the Rhynchocephalia
(Figs. 60–63)
[No manuscript. Some skull characters are noted on pages 20, 21, 25, 213, 279.]
Fig. 60. Rhynchocephalian skull: Sphenodon (Tuatera), from the side and above. pm, premaxilla; n, nasal; prf, prefrontal; f, frontal; pf, postfrontal; p, parietal; po, postorbital; sq, squamosal; m, maxilla; j, jugal; qj, quadratojugal; q, quadrate; c, coronoid; sa, surangular; art, articular; pa, prearticular; d, dentary; an, angular.
Fig. 61. Thalattosaur skull: Thalattosaurus, from above and from the side. After Merriam. One eighth natural size.
Fig. 62. Rhynchosaur skulls: A, B, Stenometopon, from above and from the side. After Boulenger. One fourth natural size. C, Hyperodapedon, from the side. After Huxley. D, the same, from above. After Burckhardt. E, from below. After Boulenger. One fourth natural size.
Fig. 63. Choristoderan skull: Champsosaurus, from above and from below.
After Brown. One half natural size.
The Skull of the Pseudosuchia
(Fig. 65 a–e)
The skull of the typical Pseudosuchia is very much like that of the Pelycosimia (Fig. 64), in structure. All the bones of the skull roof are present except the dermosupraoccipital, tabular, and supratemporal; the lacrimal is small; there is no parietal foramen; and the palate bones have the primitive relations. Other forms, however, referred to this group provisionally, have both the dermosupraoccipital and tabular (Youngina, Fig. 64 c), and teeth on the prevomers and pterygoids (Proterosuchus). The upper and lateral temporal openings, a large antorbital vacuity and one in the mandible, are like those of the Parasuchia. The antorbital foramen is large, as are also the orbits. The supratemporal foramen is large and never posterior in position.
The Skull of the Pelycosimia
(Fig. 64)
The skull of the Pelycosimia differs from that of the Phytosauria chiefly in the position of the external and internal nostrils near the extremity of the face, and at a considerable distance in front both of the orbits and antorbital openings. The face is short in front of the nostrils. There is also no respiratory channel back of the internal nostrils, so characteristic of the phytosaurs. The skull is markedly carnivorous in type.
Fig. 64. Pelycosimian skull: Erythrosuchus, from above. After von Huene. One sixth natural size.
The Skull of the Phytosauria
(Figs. 65, 66, 67)
The skull of the Phytosauria is nearly uniform in general structure, characterized especially by the elongated face and posterior location of the external nostrils. No bones are fused in the midline, and none, save the primitive dermosupraoccipital, tabulars, and supratemporals are missing. The paroccipitals, as usual, are firmly fused with the exoccipitals. There is no parietal foramen. The supratemporal openings are more or less depressed below the level of the parietals but retain their primitive boundaries. The well-developed quadratojugals enter into the formation of the lateral temporal openings posteriorly. There is a primitive quadrate foramen between the quadratojugal and the quadrate. The stapes is slender. There is a large antorbital foramen bounded by the maxilla, nasal, lacrimal, and jugal.
The greatly elongated face is composed chiefly of the premaxillae, which extend back to the anterior ends of the nares, with the septomaxillae intervening, in the middle. The nostrils are surrounded by the large nasals and are elevated to or above the superior plane of the skull.
The bones of the palate retain their primitive relations, and there are small posterior palatine vacuities, larger in the more primitive forms. The pterygoids meet broadly in the median line, forming the roof of a deep respiratory channel between the heavy, underarching palatines, in some almost forming an incipient secondary palate, in the phytosaurs, as in the crocodiles, doubtless caused by the large flat tongue. The interpterygoidal opening and parasphenoid are small.
The elongate prearticular of the mandible is fused with the articular. As usual in slender-jawed reptiles with a long symphysis, the splenial participates in it, an acquired character. The condition of the coronoid is not yet definitely determined, but it is doubtless present, though small. A large foramen, so generally characteristic of the Archosauria, is constant in the outer wall of the mandible between the surangular, angular, and dentary.
The teeth are numerous, set in deep sockets and confined, as in other archosaurians, to the premaxillae, maxillae, and dentaries, 
Fig. 65. Pseudosuchian skulls: A, B, Euparkeria, from the side and from above. After Broom. Five sixths natural size. C, Youngina, from above. After Broom. Three halves natural size. D, E, Ornithosuchus, from above and from the side. After Boulenger. One half natural size.
Fig. 66. Phytosaur skull: Machaeroprosopus, from above and from below.
either cylindrical throughout or partly or chiefly flattened and denticulated. The first two or three teeth on each side, especially above, are cylindrical and much elongated. The dentulous portion of the premaxillae is long, with twenty or more teeth on each side.
Fig. 67. Phytosaur skull: Mystriosuchus. pm, premaxilla; m, maxilla; na, nasal; f, frontal; p, prefrontal; l, lacrimal; pf, postfrontal; po, postorbital; pa, parietal; sq, squamosal; qj, quadratojugal; pl, palatine; t, transverse; in, internal nares; en, external nares; pt, pterygoid; bs, basisphenoid; eo, exoccipital. After McGregor.
the extent of the elevated facial carina in front of the nares, and the shape of the teeth, more slender and cylindrical in those with slender jaws, more flattened and compressed in those with a compressed and elevated face, doubtless because of the more dominant fish-eating habits of the former, the more general carnivorous habits of the latter.
The Skull of the Crocodilia
(Figs. 68, 69)
The skull of the Crocodilia invariably lacks the postfrontals, supratemporals, epipterygoids, tabulars, septomaxillae, and parietal foramen, and the paroccipital is fused with the exoccipital. The parietals and frontals are fused in the midline. The supraoccipital is a triangular bone, excluded from the foramen magnum. The quadratojugals take part in the formation of the lateral temporal opening, narrow bones between the quadrates and jugals extending forward to meet the postorbitals. The quadrates are firmly wedged in between the quadratojugals, postorbitals, parietals, exoparoccipitals, postoptics, squamosals, proötics, basisphenoid, and pterygoids, an extensive connection. The supratemporal openings are large in the early forms, small in the later ones, and almost [or entirely] obsolete in some. The lateral temporal opening is separated in the teleosaurs from the orbits by an unmodified postorbital bar immediately below the skin. In the broader-faced amphicoelian and in all the procoelian types it is a cylindrical bar with a considerable space between it and the skin. The postoptics (Fig. 69 d, as) are fully ossified, extending from the basisphenoid to the frontals. There is no ossified interorbital septum. An antorbital vacuity is often present in the teleosaurs, but only rarely has been found in the early procoelian types. The nasals may or may not separate the external nares, connecting with the premaxillae; they are divided by a cartilaginous septum in life. The nares are always at the extremity of the face, no matter how long and slender it may be. There is a eustachian canal connecting with the otic sinuses, in the median line between the basioccipital and basisphenoid.
The most important modifications of the crocodilian skull are found in the palate, distinguishing these reptiles from all others. The maxillae meet broadly in the middle line, excluding the prevomers 
Fig. 68. Crocodilian skulls: A, Teleosaurus, from above. About one fifth natural size. B, Dakosaurus, from above. After Fraas. One twelfth natural size. C, Alligatorellus, from above. After Lortet. One half natural size. D, Alligator, occiput. One half natural size. E, Hylaeochampsa, from below. After Andrews.
Fig. 69. Alligator skull: one half natural size.
from the palatal surface, except rarely just back of the premaxillae in the caimans, or in front of the pterygoids in the tomistomids. The palatines also meet in the middle line in all, sending up processes for articulation with the postorbital, and forming the floor of the respiratory canals. The prevomers form a pair of tubes above the maxillae and palatines, articulating posteriorly with a suprapalatal prolongation of the pterygoids; they are separate as usual, and do not often appear on the palatal surface. The pterygoids also meet in the middle line, in all procoelian forms completely surrounding the internal nares, which may or may not be divided by a median partition, meeting below the nasal tubes in front of them. In the early teleosaurs these openings were at the posterior border of the palatines. In the goniophilids the openings are surrounded by both palatines and pterygoids.
The pterygoids articulate posteriorly and externally with the postoptics by a narrow pillar, possibly representing the epipterygoids. There are large posterior palatine vacuities at the sides of the palatines, and, in Hylaeochampsa, an additional opening in the palate between the ectopterygoid and maxilla.
In the mandible (Fig. 69 d) the splenials meet in a median symphysis in slender jaws. The prearticular is apparently wholly absent, or fused with the angular. There is a large mandibular foramen between the angular, surangular, and dentary on the outer side, absent in the Thalattosuchia.
The Skull of the Dinosaurs (Saurischia, Ornithischia)
(Fig. 70)
[No manuscript. Skull characters noted, pages 17, 28, 32, 214, 291–296.]
Fig. 70. Dinosaur skulls: A, Tyrannosaurus. After Osborn. One eighteenth natural size. B, Diplodocus. After Holland. One sixth natural size. C, Kritosaurus. After Lambe. One twelfth natural size. D, Triceratops. After Marsh. One twelfth natural size.
The Skull of the Pterosauria
(Figs. 71, 72)
[No manuscript. Skull characters noted, pages 11, 14, 17, 19, 21, 28, 214, 296–298.]
Fig. 71. Pterosaur skulls: A, Rhamphorhynchus, from the side. B, Campylognathus, from the side. After Plieninger. One half natural size. C, Rhamphorhynchus, front part of palate. After von Huene. One half natural size. D, Ornithodesmus, end of beak. After Hooley. One fourth natural size. E, Pteranodon. About one fourteenth natural size.
Fig. 72. Pterosaur skulls: A, Nyctosaurus, from above; B, the same from below. About five eighths natural size. C, Pteranodon, from below, after Eaton. About one fourth natural size.
- ↑ [In addition to the openings noted by the author, paired subtemporal openings occur in all reptiles in the palatal aspect of the skull; they are bounded medially by the pterygoids and the basis cranii, laterally by the lower temporal bar, if present, or by the dermal covering of the temporal region, as in Squamata.—Ed.]
- ↑ [The cumulative evidence against the views of Gaupp and Jaekel, with regard to the reptilian homologue of the mammalian lacrimal, has been set forth in the Bulletin of the American Museum of Natural History, vol. XLII, pp. 99, 131–135.—Ed.]
- ↑ ["Maxillae"—a lapsus calami for "parietal"?—Ed.]
- ↑ [But see footnote, p. 52.—Ed.]
- ↑ [Watson has shown that this view is untenable.—Ed.]
- ↑ [It is separate in at least some geckos.—G. K. N.]
- ↑ [Possibly "paroccipital plate" was intended.—Ed.]
- ↑ [Broom, Sollas, Watson, and von Huene have observed a distinct quadratojugal in various Therapsida.—Ed.]
- ↑ [Also Dibamidæ.—G. K. N.]
- ↑ [Some geckos have them separate.—G. K. N.]