VERTEBRATA: REPTILIA. 531 



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 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 (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 tail can almost always be withdrawn at will under the shelter 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 : 



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

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

 which are eight in number, and are termed the "neural plates " 

 (;z). 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 



