70 ELEMENTS OF COMPARATIVE ZOOLOGY. 



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. Crocodiles and alligators are distin- 

 guished from each other by the fact that the former 

 have fully webbed feet and more slender snouts. The 

 gavials of the rivers of India have the snout even more 

 slender. The alligators 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 were the whales 

 among the reptiles of former times, while the Plesiosaurs, 

 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, having thigh- 

 bones nine feet in length and vertebras five feet across. 

 The Pterodactyls were flying reptiles with wings like those 

 of the bats, except that the wing-membrane was supported 

 by a single finger. 



