OLIGOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 223 



codus and Anthracotherium of European origin; the anthracotheres exhibit 

 no tendency to attain the great proportions displayed in the European and 

 Asiatic forms. Of the pig-hke forms the peccaries (Perchoerus) are less 

 numerous than the entelodonts; the latter now begin to attain giant size 

 {E. ingens), thus assuming a rank similar to that of the anthracotheres in 

 Europe. 



The perissodactyls are similarly differentiated into the massive river- 

 frequenting amynodonts {Metamynodon) with hypsodont molars and power- 



FiG. 110. — The Lower Oligocene three-toed horse Mesohippus, a swift, light-limbed animal. 

 To the right Dinictis, the light-limbed saber-tooth cat. After original by Charles R. Knight 

 in the American Museum of Natural History. 



ful canine tusks, and the extremely long-limbed, long-footed, but still tri- 

 dactyl lophiodonts (Colodon); these lophiodonts are of considerably larger 

 size than the contemporary horses. From the Oreodon Zone comes the 

 typical Mesohippus hairdi of Leidy, an extremely light-limbed equine, and 

 there now is becoming more apparent the incipient adaptive radiation of 

 the horses into forest-living and browsing types {Mesohippus eulophus, 

 remotely related to the forest-living horse, Hypohippus, of the Miocene), 

 and plains or grazing types (M. obliquidens) . Some of these horses are 

 chiefly found in the 'Clays' {M. eulophus, M. hairdi, M. obliquidens), others 

 chiefly occur in the 'sandstones' of this and higher levels {M. intervicdius, 

 M. validus, M. gidleyi). The tapirs (Protapirus) are rare. The fleet-footed 

 cursorial rhinoceroses, or hyracodonts, are numerous and characteristic 

 of this horizon. As in Europe the true rhinoceroses are clearly divided 

 into the dicerathere, or two-horned, and the acerathere, or hornless, series. 



