MAMMALIA 269 



ous pouches on the wall of the stomach, enable the camel 

 to go many hours without water in the sandy Sahara. The 

 tusks of the elephant are the two upper overgrown incisor 



FIG. 305. Wapiti or American elk. Photographed in the Philadelphia Zoological 

 Gardens. 



teeth composed almost entirely of dentine, which furnishes 

 the ivory of commerce. 



Gnawers (Rodentia). Any one of the four hundred 

 species of rodents living in America may be recognized by 

 the large chisel-shaped incisor teeth, of which there are two 

 in the lower jaw and either two or four in the upper one, 

 while the canine teeth are wanting (Fig. 309). 



