ORDO IT. CHELONIA. 



WE come now to the last order, that of the Chelonians, which stands 

 at the head of the class of Reptiles. Turtles seem to be the link, the 

 transition, between their class and the higher classes of vertebrata : 

 the Birds and Mammals. 



Nothing more easily recognizable than a reptile of the Chelonian 

 order : a double shield, one more or less convex or arched, is on the 

 back, and known as the carapax ; the other is flat or nearly so, opposed 

 to the former, and called the plastron or sternum, both so combined as 

 to constitute a kind of inflexible box or trunk, between the extremities 

 or edges of which, the head, limbs, and tail, may, in most cases, be 

 either entirely or partly retracted or withdrawn. 



In all vertebrata, the solid frame is composed of a series of subcylin- 

 drical bones, the vertebrae, constituting a flexible chain in the direction 

 of the longitudinal axis of the body, and on the sides of which the 

 ribs are inserted, whilst the sternum, or breast bone, is placed under 

 the latter, forming together a cavity, in which are found the principal 

 viscera of the animal economy, the flesh or muscles enveloping the 

 bony frame. In the Chelonians, the dorsal vertebrae and the ribs 

 expand, and unite more or less intimately together, to form the rigid 

 carapax and the sternum or plastron, both being protected exteriorly 

 either by the skin alone, or else by horny and epidermic shields or 

 large scales ; the muscles as well as the viscera being lodged inte- 

 riorly, the muscles along the inner surface of the bones, the viscera 

 in the cavity proper. 



The Chelonians have no teeth properly so to be called ; the jaws 

 are generally stout and robustly built, protected by a horny sheath, 

 constituting a bill, in shape not unlike that of some Parrots and birds 

 of prey, their edge being, however, occasionally serrated. The palate is 

 toothless also. As to the tongue, it is thick and fleshy, freely mova- 



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