160 STORY OF THE REPTILES 



family proper, but the two groups have very different 

 structure. There is quite a tendency now to classify 

 the order by the peculiar jointing of the bones of 

 the neck — usually shown in the manner by which the 

 creature tucks away its head in the shell, whether 

 by drawing it directly in or by bending it side wise. 

 Perhaps this and other anatomical features — espe- 

 cially those connected with the head — may change 

 our groups any day according to the emphasis put 

 upon any one set of peculiarities. 



The Crocodilians 



These may be roughly divided as follows : 



A. Tusks of lower jaw bite into grooves of the upper. 



(Crocodiles generally.) 

 B. Edges of jaw nearly straight ; jaws long, thin, and narrow. 



Oavial. 

 BB. Edges of jaw scalloped or wavy. 



Egyptian or True Crocodiles. 

 AA. Tusks of lower jaw bite into a pit in the upper. 



(Alligators generally.) 

 C. Plates of back not hinged ; simply connected by the 

 skin. 

 D. Bony plates on underside of body ; front toes free, 



Asiatic Alligator. 

 DD. Bony plates wanting below ; front toes webbed. 



No7'th American Alligator. 

 CC. Plates of back hinged to each other. 



Caiman or South American Alligator. 



Serpents 



Dr. Giinther has noted that in the popular mind 

 snakes are divided by habit into — 



