39 2 ZOOLOGY. 



and there are some which never come to land. These and some land 

 forms, bring forth their young alive. Many snakes are beautifully 

 and characteristically colored. In some instances the coloration is deemed 

 to be protective. 



Order IV. Crocodilia (Crocodiles, Alligators, etc.). Fresh-water rep- 

 tiles with elongated bodies bearing two pairs of well-developed appendages. 

 The skin is armed with dermal bony scales or scutes covered by epi- 

 dermal scales. Teeth occur in sockets. The quadrate is immovable and 

 the sternum is present. The adult heart is completely divided into right 

 and left halves. The cloaca opens by a longitudinal slit. Here are 

 included the gavial of the Ganges, tne crocodiles of the Nile and of tropical 

 America, and the alligator of America. They are somewhat sluggish 



FIG. 200. 



FIG. 200. Head of the American Alligator {Alligator Mississippiensis). From Eckstein. 



animals, but when hungry will attack with sutcess the larger mammals 

 or man. They may attain a length of twenty feet or more. Crocodilia 

 are chiefly aquatic, though they rest on the shore, and deposit their eggs 

 in the sand where they hatch. 



There are numerous orders of extinct reptiles which show close rela- 

 tionship with the fishes, amphibians, and birds of early geological times. 

 This is merely another way of saying that the early Reptilia and the 

 other vertebrates were much more generalized in their characteristics and 

 less differentiated than those of the present. 



. 417. Supplementary Topics for Investigation. 



1. Have the venomous snakes any characteristic appear- 

 ance? 



2. Report on the habits of the rattle-snake. Whence the 

 structure giving rise to the name? The nature of the fang 

 and the poison gland. 



3. What is the degree of activity and strength of the rep- 

 tiles and cold-blooded animals, as compared with the warm- 

 blooded? 



