MAMMALS. 113 



seas reach a length of sixty feet, the razor-back whales are 

 still larger, while the sulphur bottoms and silver bottoms 

 (so called on account of the color of the lower surface) 

 attain a length of from 90 to 95 feet. 



ORDER VI. SIREKIA (Sea-cows). 



These are whale-like animals, with the same flippers and 

 the same horizontal tail, but they differ from the whales in 

 the possession of an evident neck, and of sparse hair or 

 bristles all over the body. Besides these features all, ex- 

 cept the extinct Rytina, have flat-crowned molar teeth. The 

 living forms are very few. Rytina, which lived near Ber- 

 ing Strait, was exterminated in the last century. The 

 dugong is the representative of these forms in the Indian 

 Ocean, while the three species of manatees come, one from 

 Africa, the other two from the eastern coasts of America. 

 All the sea-cows are vegetable feeders, living upon sea- 

 weed or, in the case of the manatees, upon the plants found 

 in fresh-water streams as well. 



ORDER VII. PROBOSCIDIA (Elephants). 



The elephants are the giants among the land mammals. 

 They have five toes, each encased in its own hoof; they 

 have no incisors in the lower jaw, while the pair in the 

 upper jaw are developed into large tusks. Canines are 

 lacking, but there are seven molars in each half of each 

 jaw. These molars are flat-crowned, the surface of the 

 crown being crossed by several ridges of harder enamel. 

 Only two, or at most three, of these molars are in use at 

 once, but as the old ones wear out they drop out at the 

 front of the jaw, and are replaced by new ones from behind 

 until the seven are gone. The skull is enormous, but it is 



