174 



THE FOOD OF ANIMALS 



Behring Sea until 1768, when it became extinct, having been 

 exterminated for the sake of its flesh. The horny plates present 

 in Manatees and Dugongs were here of greater extent, to com- 

 pensate for the complete absence of teeth. 



Fig. 421. — The Dugong [Halicore dugong) 



GNAWING MAMMALS (Rodents) 



Rodents include a vast number of widely -distributed small 

 animals, of which the great majority feed entirely on vegetable food. 

 A Rabbit {Lepus cuniculus) or Hare {L. timidus) (fig. 422), illus- 

 trates very well the adaptations to a vegetable diet exhibited by 

 the members of the order. Comparatively few teeth (twenty- 

 eight) are present, the limited number being accounted for by the 

 entire absence of canines and reduction of incisors. All the teeth 

 grow continuously throughout life, in correspondence with which 

 peculiarity they do not narrow into " fangs " within the substance 

 of the jaws, but are of uniform thickness throughout. Two long 

 chisel-edged incisors are present in the front of each jaw, and it 

 is these which constitute the gnawing structures characteristic 

 of the entire order. We have seen how the complex grinding- 

 teeth of Ruminants, &c., are kept rough by being made up of 

 materials of different degrees of hardness; this is a case where 



