CHELONIA. 



357 



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. 313, r), and 



Fig. 313. Skeleton of Tortoise (Ewys Enropcea?, the plastron being removed, en 

 Carapace ; r Ribs, greatly expanded, and united by their edges ; ^ Scapular arch, placed 

 within the carapace, and carrying the fore-limbs; / Pelvic arch, also placed within the 

 carapace, and carrying the hind-limbs. 



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. 



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 



