The Tetrapoda: Amphibia, Reptiles, Birds 611 



located on top of the head; thus the crocodile or alligator can float in 

 the water with only the nostrils and eyes protruding. Old crocodiles 

 look like logs floating in a placid tropical stream. When the croco- 

 dilian submerges, special valves close off the nostrils and ears. The 

 entire body is covered with heavy scales. 



Within the United States, there are but two species of crocodilians : 

 the American alligator, Alligator mississippiensis (Fig. 202,/), and the 

 American crocodile, Crocodilus americanus. These forms are found only 

 in Florida and the southern portion of some of the Gulf States. The 

 crocodile and alligator are very similar in appearance, but the alligator 

 has a broad head rounded at the snout, while the crocodile has a trian- 

 gular head with a narrow snout. 



When young, the crocodilians feed chiefly on small fish; when 

 adults, they rely upon birds and mammals that frequent the streams 

 or swamps where they live. Most of them need not be regarded with 

 fear, for they are comparatively harmless. Two species, however, the 

 salt-water crocodile of India and Malaysia and the African crocodile, 

 are reputed to be very dangerous to human beings. 



In size, the crocodilians vary greatly. The gavial of India is 

 reported to attain lengths of 30 feet ; fortunately its food consists largely 

 of fish and it need not be feared. The black caiman of South America 

 reaches a length of 20 feet as does the salt-water crocodile. The largest 

 American forms attain lengths of about 16 feet. 



The crocodilians lay their eggs in holes in the ground or in some 

 cases construct a moundlike nest. About these nests are piled sticks 

 and vegetation, the decomposition of which provides heat for the incu- 

 bation of the eggs. 



Economically the members of this order are important as a source 

 of leather. This leather has a high value in the world's markets. In 

 some areas of Florida, alligators are raised on special "farms" for 

 their hides. 



The Order Rhyiichocephalia. — On a few small islands off the 

 coast of New Zealand are the sole surviving members of this order. 

 These are the tuateras, Sphenodon punctatitui. These strange noc- 

 turnal animals are quite lizardlike in appearance and possess a dorsal 

 spiny crest. The largest of them are about 2 feet in length. Their 

 interest to scientists is based on the fact that they possess characters 

 showing their relationship to the ancient reptilian groups which gave 

 rise to the lizards, crocodilians, and dinosaurs. One character which 



