REPTILES. 291 



strangely altered. The vertebra) have become immoveable. 

 and the ribs so widened as to touch each other throughout 

 their entire length. Still the anatomist can trace, under 

 these and other modifications of structure, the parts with 

 which he is familiar in other animals. In the lower shell, or 

 plastron (Fig. 236), he can, in like manner, recognise the 

 breast-bone (sternum), modified in its structure so as to form 

 a large oval plate. 



The number of species at present known is sixty-nine; and 

 these, arranged according to their habits, may be conveniently 

 spoken of as 



Land Tortoises, of which there are 15 species. 



Freshwater Tortoises 46 , , 



Turtles, or Marine Tortoises, 8 ,, 



The animals of this order are, more than any other reptiles, 

 limited to the warmer portions of the globe; yet three of the 

 marine species, having at different times been borne by the 

 waves and currents to different parts of the shores of these 

 countries, are, according to established custom, entitled to 

 rank with our indigenous animals. 



Among the species thus 

 added to our Fauna is the 

 Hawk's-bill Turtle* (Chelo- 

 nia imbricaia, Fig. 234). 

 The one best known to epi- 

 cures is the Green Turtle 

 (Chelonia mydas); but the 

 former species is that which 

 supplies the valuable Tor- 

 toise-shell of commerce, and 

 to it our observations must 

 be restricted. 



" The structure of the 

 whole family is admirably 

 adapted to their marine 

 habits. The body is flattened 

 so as greatly to facilitate 

 their progress through the water; the feet are formed into 

 the most perfect oars, by means of which they are propelled 



* The other two species are the Coriaceous Turtle Spliangis coriacea 

 and Chelonia caouana. 



Fig. 234. HAWK'S-BILL TURTLE. 



