vulgaris, 4 or 5 in Anolis, to 1-3 in some other iguanids, skinks and geckos. Uroplates fimbriatus has 14, and the last four pairs are separated from the dorsal portions of their ribs; similar discontinuity occurs in geckos, the median portions bearing a striking, although not fundamental, resemblance to parasternal ribs.
In the lizards with much reduced fore limbs, the sternum
loses its connexion with the ribs from behind forwards; two
Fig. 32.—Rudiments of
pectoral arch—1, of Acontias
meleagris; 2, of Typhlosaurus
aurantiacus (after
Fürbringer).
sternal ribs existing in the Tejid
Ophiodes and in the Scincoid
Acontias, one only in Pygopus,
none in Ophisaurus s. Pseudopus
and Anguis (in the latter one
rib is still connected in the
embryo). The sternum is likewise
quite free in Chirotes in spite
of its functional limbs; the sternum is still a large plate, with
a window, and ending in two long, xiphoid processes.
Lastly, the sternum has vanished without a trace, as in the snakes, in some species of Acontias, in the Anelytropidae, Dibamus and Aniella (Fürbringer). In the limbless genera of Amphisbaenidae the sternum is very much reduced; in Tragonophis alone it is still represented by a narrow transverse bar connecting the ossicular vestiges of the shoulder-girdle; in the other genera the sternum has shrunk to a pair of nodules or to a single nodule.
The pectoral or shoulder-girdle in its completest condition consists of a right and left scapula, coracoid, precoracoid and clavicles, and an unpaired interclavicle or episternum. The dorsal portion of the scapula remains cartilaginous, with or without calcification, and is usually distinguished as suprascapula. The ventral portion of the precoracoidal and coracoidal mass remains likewise more or less cartilaginous, rather unnecessarily distinguished as epicoracoid. Ossification begins near the glenoid cavity and thence spreads, eventually with the formation of a dorsal and a ventral centre. The resulting suture separates the dorsal or scapular from the ventral or coraco-precoracoidal mass. A kind of landmark, not always reliable, between coracoid and precoracoid is the exit of the supra-coracoidal nerve. The ventral margins of the coracoids articulate in tenon and mortice fashion with the antero-lateral margins of the sternum. The interclavicle, usually T-shaped, is a dermal bone and rests upon the ventral side of the girdle. The paired clavicles, sometimes fused together, rest upon the anterior end of the inter clavicle and extend transversely to the acromial process of the scapula; the detail of the attachments varies much.
The girdle is most complete in Sphenodon and in Lacertilia. In Sphenodon the coracoid forms one continuous mass with the precoracoid, without further differentiation; the clavicles are fused with the interclavicle into one T-shaped mass, the cross-arms of which are attached to the acromia by ligaments. In the lizards (except Heloderma) the much-broadened central and anterior halves of the girdle are fenestrated; the windows, always closed by membranes, are bordered by bony processes, distally by unossified cartilage. The first window to appear, or the most constant, lies between the coracoid and its precoracoid; in Anguis it is the only window, in this case not a primary feature. In other lizards, e.g. Uromastix, a second window occurs between precoracoid and scapula, and even a third window can appear in the scapula itself, causing in many Iguanidae, e.g. Amblyrhynchus (see fig. 33, ms.), the so-called mesoscapula; an analogous window within the coracoid produces the mesocoracoid, unnecessary distinctions of little morphological value considering the great variability of these fenestration's in closely allied genera.
The chameleons have lost the clavicles and the interclavicle, and the scapula, which is very slender and long, is devoid of an acromial process. The coracoid forms one mass with the precoracoid, through the middle of which passes the supra-coracoidal nerve; the coracoids articulate by their whole bases with the sternum.
Geckos possess a complete shoulder-girdle; the ventral portion shows, e.g. Hemidactylus, three pairs of windows; only one in Uroplates. In the latter the interclavicle is much reduced; the clavicles meet each other and are slender rods. In the Geckoninae and Eublepharinae the ventral halves of the clavicles are dilated and possess each a foramen; the interclavicle is cross-shaped.
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|
Fig. 33.—Sternum and Shoulder-Girdle of Amblyrhynchus subcristatus (after Steindachner). cl, clavicle; co, coracoid; h, humerus; ic, interclavicle; mc, mesocoracoid; ms, mesoscapula; pc, precoracoid; s, scapula; st, sternum. |
In the more or less limbless genera of lizards the shoulder-girdle is much reduced. In Chirotes, which still has functional fore limbs, the clavicles and the interclavicle are absent, the coracoids are not divided from the precoracoids; in the limbless Amphisbaenidae the girdle is reduced to a pair of cylindrical ossicles in Amphisbaena, Blanus and Trogonophis; no vestiges exist in Rhineura, Lepidosternon and Anops.
Foramina in the broadened clavicles occur also in various Lacertae, for instance in the Iguanid Laemanctus, in the Scincoid Trachysaurus, in Plestiodon, Zonosaurus and in Lacerta simonyi, but not in L. agilis. In Mabuia the median portions are especially broad and show each two foramina. Their presence can be of but very doubtful taxonomic value.
The girdle of the Crocodiles is considerably simplified. Scapula and coracoidae, movably united, at least in younger specimens. The precoracoid is slightly indicated by a process of the coracoid, which is perforated by the supra-coracoidal nerve near the glenoid cavity. Clavicles are absent. The interclavicle is reduced to a long, flat splint-bone, which is firmly fused on to the sternal cartilage. The Chelonian shoulder-girdle shows several very remarkable modifications. Instead of lying outside the trunk, it has been transferred into the cavity of the trunk, the carapace with the ribs covering it from the outside. An explanation of the changes implied in this transposition is still extant. Chelonians are, moreover, the only reptiles besides Pterosauria in which the scapula is attached to the skeleton of the trunk. The scapulae stand in a more or less vertical position, and their dorsal end rests against the inside of the nuchal plate, where this is sutured to the first neural and the first costal plate, a little in front of and sidewards from the first short rib. From near its ventral end the scapula sends off a long process, which converges transversely with its fellow. This process, the clavicle(!) or the precoracoid of many authors, is the acromial process, the Plesiosauri giving the clue as to how an acromion can assume such an abnormal position. The coracoid, with a suture between it and the scapula, is very long and extends horizontally backwards, not meeting that of the other side. The sternum being
