CHORD ATA — VERTEBRATA — MAMMALS 389 



found frozen in the sands and gravels of northern Siberia along 

 the Lena River. That it was adapted to a life in a cold climate 

 is shown both by its dense, woolly short hair covered with long 

 outer hair, and by the contents of the stomach, remnants of 

 present-day Siberian vegetation, (c) The Imperial elephant 

 (Elepkas wiperator), attaining a height of over thirteen feet and 

 ranging at least from Ohio to Mexico City. (Among the existing 

 African elephants the male often attains a height of over eleven 

 feet.) The mastodon (Mammut) was more of a forest dweller 

 than Elephas ; its low-crowned teeth had each two to five high 

 ridges for crushing the succulent herbage, especially the twigs of 

 coniferous trees, while the true elephant (Elephas) with its high- 

 crowned teeth, each with ten to sixteen very low ridges, can grind 

 the harder grasses of more open regions. 



(5) Embrithopoda (Upper Eocene and Oligocene of Africa). 

 A well-known genus is Arsinoitheriui7i, — large, rhinoceros-like 

 animals, some almost six feet high at the shoulders, with a pair 

 of huge, pointed, forwardly directed horns over the snout and 

 a smaller pair above the eyes. 



(6) Toxodontia (Eocene to Pleistocene of South America). 

 The molars have flattened outer walls. Toxodon of the Pleis- 

 tocene is a characteristic genus. 



(7) Litopterna (Eocene to Pleistocene of South America). 

 Some of these animals paralleled the horses in their development, 

 but their more primitive character is shown in their smaller 

 brain, less adaptive skeletons, and lower-crowned molars. The 

 number of toes varies from five to one in different genera, but the 

 third is always the longest. Examples are : Macrauchenia 

 (Pleistocene), — three toes on both fore and hind feet; Thoa- 

 therium (Miocene), — a small animal with but one toe on fore 

 and hind feet like the existing horse. 



(8) Perissodactyla (Lower Eocene to present). In these, the 

 odd-toed ungulates, the third toe on both fore and hind foot is the 

 longest (whence the name from Greek perissos, odd, + dactylos, 

 a finger). The plane of symmetry of the foot bisects the third 



