2 Field Museum of Natural History 



and rivers in tropical or subtropical countries, though 

 two species, the Nile Crocodile (which is found 

 throughout Africa), and the East Indian Crocodile, 

 are known to swim boldly out to sea. These two 

 species are also notable as the most seriously danger- 

 ous to human beings. They are all excellent and 

 powerful swimmers and secure their food either in 

 the water or from the neighboring banks. They are 

 by no means exclusively aquatic, however, the true 

 crocodiles, especially, being capable of active motion 

 on land. All come ashore to sun themselves and to 

 deposit their eggs. In size, the crocodilians are the 

 largest of living reptiles. Some of the existing forms 

 reach an occasional length of thirty feet, while the 

 largest fossil forms are estimated at about fifty feet. 



The American alligator with a maximum adult 

 size of about sixteen feet is intermediate between 

 these monsters and the smallest forms. One of the 

 South American caimans is not known to reach a 

 length of more than four feet. 



Superficially, crocodiles resemble gigantic lizards, 

 but with the exception of the general characters com- 

 mon to all reptiles, they are really widely distinct 

 from lizards. By their large size they remind us of 

 extinct dinosaurs and examination of their skulls 

 and skeletons shows that they are really more closely 

 allied to the dinosaurs than to the other groups of 

 living reptiles (turtles, lizards and snakes, and the 

 Sphenodon of New Zealand) . 



The fossil history of the Crocodilia is of great 

 interest. Their mode of life insures the preservation 

 of their remains more frequently than is the case with 

 more terrestrial animals, with the result that the 

 record of their ancestry is rather better known than 

 that of most groups of reptiles. They reached their 

 greatest development toward the close of the great 



[26] 



