SKELETON.] REPTILES 447 number, size, or shape with the epidermal shields which (save in Trionyx and Spliargis) cover them. Their condition in Sphargis is quite peculiar. In the skin of its back there are imbedded a great number of small plates flexibly united in a mosaic-like pattern. Seven longitudinal rows of these plates (one median, six lateral) differ from the rest by their large size and raised surfaces. On the ventral surface of the body there is one large thin median plate the epister- mim, together with other very slender ossifications forming a very imperfect bony framework. All these latter bony plates, however, are very imperfectly ossified, with numerous vacuities. In the Land Tortoises and Terrapins, on the other hand, we have the exoskeleton in its greatest perfec- tion. On the back there is a series of ten median plates, the first one of which is called nuchal and the last one pygal. On each side of these series and suturally connected therewith is a series of large lateral plates also suturally united together and with the median plates. Finally, a series of marginal plates suturally connected together and with the foregoing complete a bony investment of the back which is complete and continuous, and which is known as the carapace. On the ventral surface of the body nine other bony plates form another continuous shield termed the plastron. The most anterior pair correspond with the bones known as clavicles in Man and many other animals. The azygous plate between and behind them answers to the episternum of many animals. The carapace and plastron are united at the sides of the body, but are separated medianly in front and behind to allow the protrusion and retraction of the head and the four limbs and tail. In Chelonia similar plates exist, but they are less developed, and merely form an imperfect species of investment, with many vacuities between the plates. The same is the case in Trionyx, wherein the marginal plates are also wanting. The skin of the neck and limbs is covered with scales. The skin of the neck may develop fimbriated processes and caruncles, as in Chelys, and the nose may be produced into a short proboscis, as in Trionyx. True claws may be absent, as in Sjthargis, or there may be but one on each foot, as in Chelone, or two, as in Caretta, or three, as in Trionyx, or four or five, as in Testudo and Emys. The claws may approximate in form to hoofs in some of the large Land Tortoises. There may be a web between the digits, as in Emys, or the whole extremity may be united into a solid paddle, as in Chelone. In all cases the skin of the jaws is found thickened and condensed so as to form a horny beak with a cutting edge. In many Chelonians there are two pairs of glands on each side just in front of the junction of the plastron and carapace, while in the Mud Tortoises, Trionyx, there is yet another gland on each side in front of the margin of the plastron. The Internal Skeleton. The endoskeleton of Reptiles, as of most Vertebrate animals, consists of parts which are divisible into two categories : (a) those which form the skeleton of the head and trunk, i.e., the axial skeleton ; (6) the parts which form the skeleton of the limbs, i.e., the appendi- cular skeleton. THE AXIAL SKELETON. In describing the axial skeleton it will be well to begin with that part of it which belongs to the trunk, leaving the more complex skeleton of the head, i.e., the skull, for subsequent consideration.
- 'ton Skeleton of the Trunk. The backbone, spinal column,
^ or spine consists in all adult Reptiles of a series of ossi- ' fied vertebrae, many, almost all, of which are separate and not anchylosed one to another. Nevertheless the spinal or vertebral column varies in its structure much more in the class of Reptiles than even in the Mamma, lia, and very much more than in the class of Birds, with which the Reptiles are so much allied. This variation consists in differences not only as regards the number of regions or vertebral categories and the extent and struc- ture of each region, but also as regards the form of the individual vertebrae and notably the form of the vertebral centra. There may or may not be distinct cervical vertebrae with or without movable ribs. The first two vertebrae are differentiated as axis and atlas, and in front of the latter there may be a rudiment of another vertebra, which has been distinguished as the proatlas. 1 There are always dorsal vertebrae, some of the ribs of which may not, but more generally do, join a sternum. These dorsal ribs are generally movable, and may be, as in Serpents, organs of locomotion. They may, on the contrary, be firmly fixed by suture one to another as in Chelonians. There may or may not be lumbar vertebrae, and two or more (in some extinct forms many) vertebrae may unite to form a sacrum. There are always caudal vertebrae, and these generally have chevron bones beneath them. Sometimes vertebrae which are not sacral become anchylosed together, as in the dorsal vertebrae of Chelonians. Articular processes always connect together adjacent vertebrae which are not thus anchylosed, and there may be accessory articular processes peculiar to the class, and which will be shortly described later. As to the form of the vertebral centra, they may bo flat in front and behind, or biconcave (amphiccelous), or biconvex, or with a ball behind and a cup in front (pro- ccelous), .or with a ball in front and a cup behind (opistho- ccelous). In the Crocodilia all the above-mentioned regions are in Croco- distinct, there being usually 9 cervical, 11 to 12 dorsal, 2 dlhans ; to 3 sacral, and about 40 caudal vertebrae. In existing species all the vertebrae are procoelous except the atlas, axis, sacral, and first caudal vertebrae. The adjacent sur- faces of the centra of the sacral vertebrae are flat, and the centrum of the first caudal is biconvex. The atlas consists of five pieces, 2 and the odontoid bone is not anchylosed to the axis. The caudal vertebrae are elongated and com- pressed, and, except the most anterior and posterior, support chevron bones. Ribs are very generally present. Those attached to the atlas and axis are single in origin. Each rib of the other cervical vertebrae bifurcates at its upper end into a tubercular and a capitular process, which respec- tively articulate with the neural arch and centrum of their supporting vertebra, and the interval thus left in the succeeding vertebras forms a canalis centralis. The ribs of the middle five cervical vertebrae so expand distally as to impede the lateral flexion of the neck. The neural arches articulate by suture with the vertebral bodies. The dorsal ribs become attached to processes which pass out from each vertebra to the tubercular and capitular processes of the ribs respectively, and their situation with respect to the neuro-central suture changes by degrees, through the vertebral series, till in the twelfth dorsal one long trans- verse process, passing out above the neural central suture, supports both processes. Thence backwards these arti- culating surfaces approximate till at last there is but a single articular surface between each rib and its sup- porting vertebra, as was the case in those of the atlas and axis. The dorsal ribs consist of two pieces the distal piece remaining cartilaginous ; and most ribs support an imper- fectly ossified processw uncinatus, nearly as in Birds, which extends backwards from the distal portion of its ossified 1 Discovered by Prof. Paul Albrecht. 2 In front of it a rudimentary proatlas has been found.
