404 



ZOOLOGY 



Photograph by E. R. Sanborn, 

 N. Y. Zool. Soc. 



FIG. 177. Hunting dog (Lycaori) ; 

 South and East Africa. 



Ptiolograph by E. R. S 

 N. Y. Zool. Soc. 



FIG. 178. Raccoon dog (Nyctercutes pro- 

 cyonides) ; Japan and Northeast Asia. 



Rodents, 

 gnawing 

 mammals 



this genus (Odobcsnus) are modified into immense tusks, 

 which are used in digging for food and in fighting. 

 Although the animals are so large, they feed mainly on 

 bivalve mollusks which they find in the mud and sand 

 of northern shores. 



(d) Rodentia, Rodents, or gnawing animals, best 

 known by their peculiar teeth. The canine 

 teeth are absent, while the incisors grow from 

 persistent pulps, grinding against one another. 

 When, as occasionally happens, an upper in- 

 cisor is knocked out, the lower one opposed to 

 it continues to grow in a circle, eventually 

 entering the brain and killing the animal. The 

 surfaces of the grinding teeth are more or less 

 flattened, not conical as in carnivores and in- 

 sectivores. The enamel pattern is often elabo- 

 rate. Rodents are the dominant and diversified 

 mammals, in this respect corresponding to the 

 Passeriformes among the birds. They include 

 the squirrels, chipmunks, woodchucks, beavers, 

 gophers, mice, rats, porcupines, guinea pigs, 

 and many lesser-known forms. The guinea pigs 



