REPTILES. 349 



tropical seas. These plates have the peculiarity that 

 they can be united by heat, so that pieces of any desired 

 size may be obtained. While many turtles are most 

 inoffensive creatures, others, like our snapping-turtles and 

 our soft-shell turtles, are ferocious, the yn:ng snapper 

 showing its temper as soon as it is hatched h\ m the egg. 



ORDER IV. CROCODILIA (Crocodiles and Alligators). 



These forms have the highest development of brain and 

 heart of any of the reptiles, the heart being incompletely 

 four-chambered. In general shape they are closely like 

 the lizards, but in bony and other structural features they 

 are greatly different, among these being the immovable 

 quadrate. Crocodiles and alligators are distinguished from 

 each other by the fact that the former have fully webbed 

 feet and more slender snouts. The ga vials of the rivers 



FIG. 146. Restoration of Ichthyosaur (modified from Fraas). 



of India have the snout even more slender. The alli- 

 gators are confined to the New World, while the crocodiles 

 occur in both hemispheres. 



The fossil reptiles show a greater range of forms than 

 the living species. The Ichthyosaurs represented the 

 whales among the reptiles of former times, while the Plesio- 

 saurs, also swimming forms, had extremely long necks. 

 The Dinosaurs were like the birds in many structural 

 features, although they lacked powers of flight and were 

 terrestrial or aquatic. Some were enormous in size, hav- 



