250 THE HISTORY OF CREATTOX. 



half of the Thick-skinned animals — rhinoceroses, tapirs, and 

 palseotheria — manifest the closest relationships to horses, 

 and have like them odd-toed feet ; whereas the other 

 half of the Thick-skinned animals — pigs, hippopotami, and 

 anoplotheria — on account of their double-toed feet are much 

 more closely allied to ruminating animals than to the 

 former. Hence we must, in the first place, among Hoofed 

 animals distinguish the two orders of Paired-hoofs and Odd- 

 hoofs, as two natural groups, which developed as diverging 

 branches out of the old tertiary primary group of Primary 

 Hoofed animals, or Prochela. 



The order of Odd-hoofed animals (Perissodactyla) com- 

 prises those Ungulata in which the middle (or third) toe of 

 the foot is much more strongly developed than the others, 

 so that it forms the actual centre of the hoof This order 

 includes the very ancient, common, primary group of all 

 Hoofed animals, that is, the Primary-hoofed animals (Pro- 

 chela), which are found in a fossil state in the oldest Eocene 

 strata (Lophiodon, Coryphodon, Pliolophus). Directly allied 

 to this group is that branch which is the actual primary 

 form of the Odd-hoofed animals, namely, the Palceotheria, 

 fossils of which occur in the upper Eocene and lower 

 Miocene. Out of the Palseotheria, at a later period, the 

 rhinoceroses (Nasicornia) and rhinoceros-horses (Elasmo- 

 therida) on the one hand, and the tapirs, lama-tapirs, and 

 primseval horses, on the other, developed as two diverging 

 branches. The long since extinct primseval horses, or 

 Anchitheria, formed the transition from the Paleeotheria 

 and tapirs to the Miocene horses, or hipparions, which 

 are closely allied to the genuine living horses. 



The second main group of Hoofed animals, the order of 



