ANATOMY.] REPTILES 445 body and large size the marine Reptiles form the nearest approach in nature to the modern creature of imagination, the " Sea-Snake." There is no question that they deviate more from the Lacertilian type than any of the other fossil forms mentioned above, especially in some of their cranial characters, which are more Ophidian. Hence Cope placed them into a distinct order of Reptiles, Pythonomorpha. Their body was covered with osseous scutes. Besides Mosasaunis, remains of which have been known and described since the year 1766, a number of other genera from Cretaceous rocks of Europe and North America have been distin- guished by Owen, Cope, Marsh, and-Dollo : Liodon, Clidastes(fig. 2), Sironedes, Platccarpus, Baptosaurus, Diplotomodon, Edestosaurus, Holosaurus, Lcstosaurus, Tylosaurus, Pterycollasaurus, Plioplate- carpus. Order 9. OPHIDIA. Snakes. Vertebrae procoelous, extremely numerous ; no sacrum ; ribs single-headed. No chevron bones on any of the vertebrae. Not only the quadrate bone is movably articulated to the skull, but also the suspensorium and the bones of the palatal maxillary apparatus are movable ; brain capsule entirely osseous. No quadrato-jugal arch. No foramen parietale. Rami of the mandible united by ligament. No trace of anterior extremities, and posterior only sometimes rudi- mentally indicated. Copulatory organs paired ; urinary bladder absent. Integuments folded into regularly ar- ranged scales. For the numerous recent genera see SNAKES. Fossil forms are scarce, and do not appear before the Eocene (Laophis, Paleeophis, Palei~yx). Order 10. CHELONIA. Tortoises and Turtles. Cervical and dorsal vertebrae not numerous. The dorsal vertebrae and expanded ribs (with the exception of Sphargis) are united into a carapace, the elements of which are immov- able, and which is completed ventrally by a number of dermal bones, a true sternum being absent and replaced by a plastron. All the bones of the skull are suturally united, with the exception of the mandible and hyoid; the dentary portion of the mandible consists of one bone only. Pectoral arch consisting of the scapula, with which the precoracoid is united, and the coracoid. Clavicles are represented by the anterior elements of the plastron. The pelvis consists of the usual bones, but is not attached to a sacrum. Two pairs of limbs. No teeth, these being replaced by horny sheaths of the jaws. Copulatory organ single. Integuments consisting of horny scutes covering the carapace, and of scales and tubercles on the soft parts. For the numerous living genera see TORTOISES. Kemains of extinct Tortoises are found from the Trias downwards, but they do not show any approximation to some other Reptilian type, or indicate a successive development. The most generalized type, Sphargis, is not older (according to present evidence) than some of the more specialized genera, its earliest representative being the remarkable Protostega from North-American Cretacean formations. Some of the Tertiary fossils exceeded in size the largest of living forms, such as the Plimalayan Colossochelys, the German Macrochelys, the North-American Atlantochdys. (A. C. G.) THE ANATOMY OF REPTILES. As the principal features known of the anatomy of extinct Reptiles have been sufficiently noticed in the several separate articles devoted to them, this chapter will deal almost exclusively with the general structure of living forms. Inasmuch as the class of Reptiles is one of the classes which make up that great primary zoological division known as "vertebrate animals," they of course possess all those structural characters which are common to that division (see VERTEBRATA). They also possess in common a certain number of structures which they share with Birds (see SAUROPSIDA), and which will be indicated in our Geneial notice of the different sets, or systems, of organs which cnar - compose the bodies of the animals of which this article acters - treats. Every Reptile has a body made up of a head, a trunk, and a tail, though, as in some Lacertilia and many Ophidia, these regions are not marked off one from another by any constriction or noticeable alteration of diameter. The posterior aperture of the alimentary tube always marks the termination of the trunk and the commencement of the tail. In some kinds of Reptiles as, e.g., in the genera Anguis and Amphisbsena amongst the Lacertilia, and in such forms as Typhlops and Uropeltis amongst the Ophidia the whole body consists of little more than a very elon- gated trunk with a small head at one end and a short or even quite rudimentary tail at the other. A neck may be interposed between the head and the trunk; this, how- ever, is generally short, as in the Lacertilia and Crocodilia, but may be more or much elongated, as in the Chelonia. It was extraordinarily long in the extinct Sauropterygia like that of a Swan. The head may be very large, as in the Crocodilia and extinct Ichthyo2)terygia, or small, as in the Sauropterygia, or very small indeed, as in Typhlops and Uropeltis. It always contains the organs of taste, smell, hearing, and sight, but there may be, as in many Lizards and all Snakes, no external indication of an ear, and the eyes are almost hidden by the skin in Snakes such as Typhlops, and certain Lizards such as Amphisbaenians and some Skinks. The mouth may be very large, as in the Crocodiles, or very small, as in Typhlops. The trunk may be exceedingly elongated, as in the instances just above referred to, or relatively very short and broad, as in the Chelonia. The tail may vary in development from its rudimen- tary condition in Typhlops to a length which exceeds that of the body several times, as in not a few Lacertilia. Its distal end may be prehensile and form an important grasping organ, as in almost all Chamaeleons. Besides these regions, there are generally two pairs of limbs,' one pectoral, the other pelvic, though these may be altogether wanting as far as regards any external mani- festation, as in all Ophidians and certain Lacertilians like Anguis and Ophisaurus. Internal rudiments of limbs may, however, be present when there is no external indication of them, as will be pointed out when describing the appen- dicular skeleton. There may be but one pair of limbs, and these pectoral, as in the Lizard Chirotes; or there may be but one pair, which are pelvic, as in the Lizards Pseudopus, Lialis, and Ophiodes. The pectoral and pelvic limbs are, as a rule, pretty equal in development, and they may be very much so, as in Chelonians and the Sauropterygia. Both may be exceedingly small, as in many Lizards, such as a number of the Scincidse, or both may be relatively large, as in Cheloni- ans. In no existing Reptile with four limbs does either pair very greatly exceed the other in length and size, but in extinct Dinosauria the pelvic limbs were greatly in excess, while the reverse was the case with the extinct Ornithosauria. The extremities never terminate in more than five distinct digits, and the number may be reduced to one in front, as in Rhodona, or one behind, as in Dibamus. The extremities may end bluntly and be undivided, as in land and marine Chelonians, and as in the extinct Ichthyop- terygia and Sauropterygia. The pectoral and pelvic limbs are generally not very divergent in form and structure, and they may be wonderfully alike, as in the existing Tortoises and in the extinct Ichthyopterygia and Sauro-
