VERTEBRATA 159 



tail ; and four legs, with feet more or less webbed, and 

 having five toes in front and four behind. The existing 

 species are confined to tropical rivers, and are carnivo- 

 rous. The eggs are covered with a hard shell. 



There are three representative forms: the gavial of 

 the Ganges, remarkable for its long snout and uniform 

 teeth ; the crocodiles, mainly of the Old World, whose 

 teeth are unequal, and the lower canines fit into a notch 

 in the edge of the upper jaw, so that it is visible when 

 the mouth is closed ; and the alligators of the New 

 World, whose canines, in shutting the mouth, are 

 concealed in a pit in the upper jaw. The toes of the 

 gavials and crocodiles are webbed to the tip ; those of 

 the alligators are not more than half webbed. 



In the mediaeval ages of geological history, the class 

 of reptiles was far more abundantly represented than 

 now. Among the many forms which geologists have 

 unearthed are numerous gigantic saurians, which can- 

 not be classified with any of the four living orders. 

 Such are the Ichthyosaurus, Plesiosaurus, Pterodactylus, 

 Megalosaurus, and Iguanodon. 



CLASS V. Aves 



Birds form the most clearly defined class in the whole 

 animal kingdom, and in some respects are the most 

 highly specialized of the craniata. The eagle and hum- 

 mer, the ostrich and duck, widely as they seem to be 

 separated by size, form, and habits, still exhibit one 

 common type of structure. On the whole, birds are 

 more closely allied to reptiles than to mammals. In 

 number, they approach the fishes, ornithologists having 

 determined eight thousand species, or more. 



A bird is an air-breathing, egg-laying, warm-blooded, 

 feathered vertebrate, with two limbs (legs) for perching, 

 walking, or swimming, and two limbs (wings) for flying 



