2 THE BIOLOGY OF THE FROG chap. 



ally keep in a damp habitat. Very few of them can endure 

 any considerable amount of drought. Even when they live 

 far away from water during most of the year, these animals 

 nearly always go to the water in the spring to deposit their 

 eggs. 



Amphibians are confined to the torrid and temperate 

 zones. In the temperate zone they hibernate during the 

 winter, crawling into the mud or other sheltered localities 

 out of the reach of frost. In higher latitudes, where on 

 account of the extreme cold the ground becomes frozen 

 in the winter to a depth of several feet, the Amphibia are 

 absent, since they could not easily get into situations where 

 they could escape being frozen. 



The Amphibia may be divided primarily into the Stego- 

 cephali, or Labyrinthodonts, and the Lissamphibia. The 

 former group consists exclusively of fossil forms which first 

 appear in the Carboniferous era and extend into the Upper 

 Triassic. The bodies of the Stegocephali were usually 

 covered with scales and bony plates, and the skull was 

 provided with numerous dermal bones. Some of these 

 extinct animals were of very large size, and in many species 

 the dentine of the teeth was folded in a very complicated 

 manner, — a feature which caused the name Labyrinthodont 

 to be given to the group. 



The Lissamphibia lack the extensive dermal skeleton of 

 their extinct predecessors. Their tooth structure is simple 

 and they never attain large proportions. They are com- 

 monly divided into three orders which may be determined 

 by the following key : — 



Legs absent Apoda. 



Legs present. 



Tailed amphibians . Urodela. 



Tailless amphibians . .... Anura. 



