DIVISIONS OF REPTILES. 449 



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 vertebrae, with 

 the exception of the first, are immovably connected together, 

 and are devoid of transverse processes. The ribs are greatly 

 expanded (fig. 194, 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 uri- 

 nary 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 sepa- 

 rate 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 Chelydidce, and Trionycidce, 

 however, the horny jaws are covered with soft skin, constituting 

 a kind of lips. As regards the second of these points, the bony 

 case 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 almost always be withdrawn at will under the shelter of the 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 is composed of the following 

 elements : 



i. The spinous processes of the dorsal vertebra, which are 

 much flattened out laterally and form a series of broad plates. 



2 F 



