The first order of living Reptiles is that of the Chelonia, comprising the Tortoises and Turtles, and distinguished by the following characters: - There is an osseous exoskeleton which is combined with the endoskeleton to form a kind of bony case or box in which the body of the animal is enclosed, and which is covered by a leathery skin, or, more usually, by horny epidermic plates. The dorsal vertebra, with the exception of the first, are immovably connected together, and are devoid of transverse processes. The ribs are greatly expanded (fig. 290, r), and are united to one another by sutures, so that the walls of the thoracic cavity are immovable. All the bones of the skull except the lower jaw and the hyoid bone are immovably united together. There are no teeth, and the jaws are encased in horn so as to form a kind of beak. The tongue is thick and fleshy. The heart is three-chambered, the ventricular septum being imperfect. There is a large urinary bladder, and the anal aperture is longitudinal or circular. The lungs are voluminous, and respiration is by swallowing air, as in the Frogs. All will pass prolonged periods without food, and will live and move, even for months, after the removal of the entire brain (Redi).
Of these characters of the Chelonia, the most important and distinctive are the nature of the jaws, and the structure of the exoskeleton and skeleton. As regards the first of these points, the lower jaw in the adult appears to consist of a single piece, its complex character being masked by anchylosis. The separate pieces which really compose each ramus of the jaw are immovably anchylosed together, and the two rami are also united in front by a true bony union. There are also no teeth, and the edges of the jaws are simply sheathed in horn, constituting a sharp beak. In the Chelydidae and Trionycidae, however, the horny jaws are covered with soft skin, constituting a kind of lips. As regards the second of these points, the bony case (fig. 290) in which the body of a Chelonian is enclosed consists essentially of two pieces, a superior or dorsal piece, generally convex, called the "carapace," and an inferior or ventral piece, generally flat or concave, called the "plastron." The carapace and plastron are firmly united along their edges, but are so excavated in front and behind as to leave apertures for the head, tail, and fore and hind limbs. The limbs and tail can almost always be withdrawn at will under the shelter of the thoracico-abdominal case formed in this way by the carapace and plastron, and the head is also generally retractile.
The carapace or dorsal shield (fig. 291) is composed of the following elements:
1. The spinous processes of the dorsal vertebrae, which are much flattened out laterally and form a series of broad plates, which are eight in number, and are termed the "neural plates " (n). 2. The ribs (r r) are united with broad and flattened plates of bone (c' c), which are connected with one another by lateral sutures, and are known as the " costal plates." In some cases, however, the costal plates, instead of being united by the whole of their lateral margins, leave marginal apertures towards their extremities, and these openings are simply covered by a leathery skin or by horny plates. 3. The margin of the carapace is completed by a series of bony plates, which are called the "marginal plates" (fig. 291, m m). These are variously regarded as being dermal bones belonging to the exoskeleton, or as being endoskeletal, and as representing the ossified cartilages of the ribs (in this last case the marginal plates would correspond with the "sternal ribs" of Birds). Of these marginal plates the one in the middle line of the carapace in front is known as the "nuchal" plate, and is larger than the rest, while the corresponding plate behind is termed the "pygal" plate (see fig. 293, nu and py).
Fig. 290. - Skeleton of Tortoise (Emys Europaea), the plastron being removed. ca Carapace; r Ribs, greatly expanded, and united by their edges; s Scapular arch, placed within the carapace, and carrying the fore-limbs ; p Pelvic arch, also placed within the carapace, and carrying the hind-limbs.
The "plastron" or ventral shield (fig. 292) is composed of nine bony pieces, of which eight are in pairs, and the ninth is odd. Of the paired pieces, the anterior are the episternals, the middle pairs are the hyosternals and hyposternals, and the hinder pair are the xiphisternals; while the unpaired piece is termed the entosternal (fig. 292 s). The precise nature of the bones of the plastron is still a matter of doubt. Some regard them as wholly corresponding with the sternum or breast-bone; others regard them as wholly integumentary; while others, again, hold - what is doubtless the correct opinion - that the plastron is formed partly of bones belonging to the endo-skeleton proper and representing the sternum, in part at any rate, and partly of integumentary ossifications.
Fig. 291. - Transverse section of the skeleton of Chelone midas in the dorsal region. c Body of one of the dorsal vertebrae; n Expanded spinous process or "neural plate " of the same; r r Ribs; c' c' "Costal plates;" m m Marginal plates; p p Lat-eral elements of the plastron. (After Huxley.)