TRUE REPTILES. 



213 



FIG. 263. Skeleton of the tortoise, from below, j, joints of the backbone 

 grown together ; r, ribs formed into a solid cover ; s h, shoulder-bones ; 

 //, hip-bones covered by carapace, which has grown over them ; //, plas- 

 tron, or under cover. 



the limbs are adapted to aquatic or terrestrial life, as the 

 case may be. About forty species are known in North 

 America, north of Mexico. 



Marine Turtles (Chdoniida}. This family has a 

 wide distribution in warm and tropical seas ; five spe- 

 cies are known. 

 The leather tur- 

 tle, or Sphargis 

 (Fig. 264), is 

 the rarest and 

 largest known. 

 The shell is a 

 thick, leathery 

 skin, composed 

 of six longitudi- 

 nal plates, form- 

 ing raised ridges. FlG - 264. Leather turtle (Sphargis). 



