4 Field Museum of Natural History 



crocodilians may be ascribed to similar habits and a 

 similar need for mechanical efficiency. In the living 

 forms (of the genus Crocodylus) this elongate snout 

 is developed independently by three species, one in 

 Australia, one in South America, and one in Central 

 Africa. This is an example of what is called parallel 

 evolution which is usually found in structures, like 

 the slender snouts in question, definitely adapted to 

 some special use. (Fig. 1). 



How to distinguish an alligator from a crocodile 

 is a question frequently asked. The greater breadth 

 of snout which characterizes the alligator is illus- 

 trated above (in Fig. 1). The fourth lower tooth 

 which is enlarged and fang-like in all crocodiles, fits 

 into a pit inside the margin of the upper jaw in alli- 

 gators and caimans, while in crocodiles it fits into a 

 notch at the side of the upper jaw. From a side view, 

 then, with the mouth closed, this enlarged tooth is 

 concealed in alligators and caimans and visible in 

 crocodiles. (Fig. 2). Alligators are distinguished 



Fig. 2. 



Side view of head of young American Alligator and Crocodile. 

 Note the exposed lower tooth in the Crocodile. 



by a special bridge of bone dividing the nasal opening 

 of the skull, which is absent in caimans and crocodiles. 

 The appearance of the American alligator is fa- 

 miliar to everyone. Alligators are probably adult at 

 a length of about eight feet. They continue to grow, 

 however, after reaching this size, so that very old 

 specimens reach much larger dimensions. The 

 largest recorded size may be placed at about fifteen 

 feet, but even twelve-foot individuals are now ex- 



[28] 



