456 MANUAL OF ZOOLOGY sect. 



deep and ten inches in diameter, and having covered them 

 over, leave them to hatch. 



The crocodiles and alligators, the largest of living reptiles, 

 are in the main aquatic in their habits, inhabiting rivers, 

 and, in the case of some species, estuaries. Endowed with 

 great muscular power, these reptiles are able, by the move- 

 ments of the powerful tail and the webbed hind-feet, to 

 dart through the water with lightning-like rapidity. By 

 lying in wait motionless, sometimes completely submerged 

 with the exception of the extremity of the snout bearing 

 the nostrils, they are often able by the suddenness and 

 swiftness of their onset to seize the most watchful and timid 

 animals. In the majority of cases the greater part, and in 

 some the whole, of their food consists of fishes ; but all the 

 larger and more powerful kinds prey also on birds and 

 mammals of all kinds, which they seize unawares when they 

 come down to drink or attempt to cross the stream. 

 On land their movements are comparatively slow and 

 awkward, and they are correspondingly more timid and 

 helpless. 



The Crocodilia are all oviparous, and the eggs, as large in 

 some species as those of a goose, are brought forth in great 

 numbers (sometimes ioo or more), and either buried in the 

 sand or deposited in rough nests. 



CLASS V. AVES 



In many respects birds are the most highly specialised of 

 Craniata. As a class they are adapted for aerial life, and 

 almost every part of their organisation is modified in ac- 

 cordance with the unusual environment. The non-conduct- 

 ing covering of feathers ; the modification of the fore-limbs 

 as wings, of the sternum and shoulder-girdle to serve as 



