190 AXIMAL FORMS 



arisen and united on the ventral surface to form the plas- 

 tron. In this process the shoulder- and hip-girdles which 

 attach the limbs come to be withdrawn into the body, and 

 we have the curious example of an animal enclosed within 

 its back-bone and ribs. This is even more the case with 

 the box-turtles (Fig. 113), common in the eastern United 

 States, whose ventral plate is hinged so that after the 

 limbs, head, and tail have been withdrawn it may be made 

 to act like a lid to completely enclose the fleshy parts of 

 the body. 



Scales and horny plates are present, as in other reptiles, 

 the former covering all parts of the body except the cara- 

 pace and plastron, which support the plates. In nearly all 

 species the latter are of considerable size, and in the tor- 

 toise-shell turtles are valuable articles of commerce. They 

 also are sculptured in a fashion characteristic of each spe- 

 cies, and may, like the colors of other animals, render them 

 more like their surroundings, and consequently incon- 

 spicuous. 



179. Crocodiles and alligators (Crocodilia). The alligators 

 (Fig. 114) and crocodiles are much more complex in struc- 

 ture than the lizards, though their general form is much the 

 same. The body is covered with an armor of thick bony 

 shields and horny scales. These, along the median line, are 

 keeled, and extending along the length of the laterally com- 

 pressed tail form an efficient swimming organ and rudder. 

 The mouth is of large size, and is bounteously supplied with 

 large conical teeth, which are set in sockets in the jaw, and 

 not fused with it, as in many of the lizards. The nose and 

 ears may be closed by valves to prevent the entrance of 

 water, and a similar structure blocks its passage beyond 

 the throat while the mouth is open. When large animals, 

 such as hogs or calves, are captured as they come to drink, 

 these devices enable the alligator or crocodile to sink with 

 them to the bottom and hold them until drowned. The 

 limbs, short and powerful, are efficient organs of locomo- 



