190 ANIMAL 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attach the limbs come to be withdrawn into the body, and 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limbs, head, and tail have been withdrawn it may be made 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- 
