178 VERTEBRATE ANIMALS OF THE UKITED STATES 



bo Teeth not set in alveoli; anus a transverse slit. 



Ci Two pairs of limbs present (rarely absent); eyelids, 

 external ear-opening and tympanum usually present; 



lizards 2. Lacertilia (p. 179). 



Co Limbs absent; eyelids, external ear-opening and tym- 

 panum absent; snakes 3. Serpentes (p. 196). 



ao Jaws without teeth; turtles 4. Testudinata (p. 225). 



Order i. Crocodilia. — The crocodiles and alligators are elongated, 

 aquatic reptiles of large size, whose bodies are covered with scales and 

 plates and which have two pairs of short legs and a laterally compressed 

 tail. There are five digits on the fore leg and four on the hind leg, the 

 latter being webbed and the former usually so. Only the three inner 

 digits of each foot have claws. The scales are arranged in transverse 

 rows, the dorsal ones having high keels and being supported beneath by 

 corresponding plates of dermal bone. The head is compactly built; 

 the quadrate bone is immovable and the palate is bony. The jaws are 

 powerful and the teeth are set in alveoli. The tympanum is exposed, 

 but is protected by a movable fold of the skin. The nostrils lie close 

 together and each can be closed by a valve. The eye has an upper 

 and a lower lid and a nictitating membrane. The order contains a 

 single family. 



Habits and Distribution. — The Crocodilia Uve in streams and 

 ponds in the warmer parts of the earth. The American crocodile and 

 the marine crocodile of the East Indies also enter salt water, and may be 

 found far from land. All are carnivorous and feed on all kinds of verte- 

 brate animals. They are oviparous, the eggs, which have a hard shell 

 and somewhat resemble goose eggs, being laid in large nests, where they 

 are guarded by the female while the sun incubates them. Most species 

 hibernate or aestivate in the mud for two or three months or more each 

 year. 



Family Crocodylidae. — With the characters of the order: about 8 

 genera and 25 species, of which 2 genera and species occur in the United 

 States. 



Key to the Genera of United States Crocodilia 



ai Snout broad (Fig. 98), the closed mouth not showing the fourth 



mandibular tooth i. Alligator. 



a2 Snout pointed; fourth mandibular tooth exposed 2. Crocodylus. 



I. Alligator Cuvier (Fig. 98). Head broad; snout blunt; fourth 

 mandibular tooth fits into a pit in the upper jaw; nasal aperture sepa- 

 rated by a bony septum: 2 species, one in the Yang-tse-kiang, China. 



