398 MANUAL OF ZOOLOGY. 



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 are 

 immovably connected together, and are devoid of transverse 

 processes. The ribs are greatly expanded (fig. 155, 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 heart is three-chambered, 

 the ventricular septum being imperfect. There is a large uri- 

 narylbladder, and the anal aperture is longitudinal or circular. 



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. 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 in- 

 ferior 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 The limbs and tail can almost always be withdrawn at will 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. The ribs, which are also much flattened and expanded, and 

 constitute what are known as the "costal plates" (fig. 155, r). 

 They are generally eight in number on each side, and are 

 commonly united throughout the whole of their lateral margins 

 by sutures. In some cases, however, they leave marginal 

 apertures towards their extremities, and these openings are 



