REPTILIA. 177 



ORDER I. 



CHELONIA. 



The Chelonia, better known by the name of Tortoises, have a 

 heart composed of two auricles, and of a ventricle divided in two 

 unequal cavities, which communicate with each other. 



These animals are distinguished at the first glance by the double 

 shield in which the body is enveloped and which allows no part to 

 project except their head, neck, tail, and four feet. The shell (or 

 upper shield) is formed by the ribs, of which there are eight pair, 

 widened and reunited by denticulated sutures, and with plates ad- 

 hering to the annular portion of the dorsal vertebra, so that all these 

 parts are rendered fixed and immovable. The inferior shell is 

 formed of pieces, usually nine in number, analogous to a sternum. 

 A frame composed of bony pieces, which have been considered as 

 possessing some analogy with the sternal or cartilaginous portion of 

 the ribs, and which in one subgenus always remains in a cartilagi- 

 nous state, surrounds the shell, uniting and binding together all the 

 ribs which compose it. The vertebras of the neck and tail are con- 

 sequently the only ones which are movable. 



The lungs have considerable extent, and are situated in the same 

 cavity with the other viscera. The thorax, in most of them, being 

 immovable, it is by the play of its mouth that the Tortoise respires, 

 which it effects by keeping the jaws closed, and alternately raising 

 and depressing the os hyoides. The former of these motions per- 

 mits air to enter through the nostrils, the tongue then closes the in- 

 ternal orifice of those apertures, when the latter forces the air into 

 the lungs. 



Tortoises have no teeth; their jaws are invested with horn like 

 those of Birds; the Chelydes excepted, where they are covered with 

 skin only. 



They possess great tenacity of life, and instances are on record 

 in which they have been seen to move for several weeks after losing 

 their head. They require but little nourishment, and can pass whole 

 months and even years without eating. 



The Chelonia were all united in the genus 

 X 



