VERTEBRATA. 255 



The Sea-turtles, as the edible Green Turtle and the 

 Hawk's-bill Turtle, which furnishes the tortoise-shell of 

 commerce, have the limbs formed for paddles. The 

 fresh-water forms, as the Snapping Turtle, (Chelydra^) 



FIG 152. Skeleton of Turtle. 



are amphibious, and have palmated feet. Land Tor- 

 toises (Testudo) have short, clumsy limbs, fitted for slow 

 motion on land. 



4. Crocodilia, or Crocodiles, Alligators, and Gavials. 

 (Fig. 153.) These are the largest Reptiles. They have 

 a double exoskeleton, one of horny scales, (epidermic,) 

 and another of bony plates, (dermal.) The bones of the 

 skull are united by sutures, and the jaws are furnished 

 with numerous teeth implanted in distinct sockets. 

 Crocodiles can be distinguished from Alligators by the 

 fourth tooth on the lower jaw being larger than the rest, 

 and by its projecting on each side of the snout. The 

 toes of Gavials and Crocodiles are fully webbed, but 

 those of Alligators are only half- webbed. Some Croco- 

 diles in the Nile attain to a length of twenty-five feet. 



