52 CROCODILIDA. 



9. The tail is longer than the body, stout, compressed at the sides, covered with 



square plates, verticillated; those above are so carinated as to form a strong, 

 deeply serrated crest, double at its base and single towards its tip. 



10. There are four extremities — the anterior with five fingers, the posterior with 

 four toes, either palmated or semi-palmated. Both anterior and posterior are 

 armed each with three nails. 



The family Crocodilida, thus characterized, embraces three genera, viz: Alli- 

 gator, Crocodilus, and Gavialis; of these the first only is found in the United 

 States. 



