418 



GENERAL ZOOLOGY 



modern times these animals have been hunted so persist- 

 ently for their ivory that there is danger of their being ex- 

 terminated. Steps are now being taken to prevent their 

 complete destruction. 



Gnawing Mammals. The Roden'tia (Lat. rodere, to gnaw) 

 are the most numerous in point of species and are the most 

 widely distributed of all the mammals. Here belong the 



FIG. 223. Photograph of Frzewalsky's Horse 



hares, guinea-pigs, porcupines, mice, rats, beaver, woodchuck, 

 prairie-dog, and squirrels. Rodents are distinguished by the 

 absence of canine teeth and the presence of chisel-shaped 

 incisors, which grow from persistent pulps (see Fig. 212). 

 They are mostly terrestrial animals, though a few, like the 

 beaver, are modified for an aquatic existence, and others, like 

 the squirrels, for life in the trees. 



The hares are distinguished from all other rodents by the 

 presence of a second pair of incisor teeth behind the first pair 



