HISTORY OF THE PERISSODACTYLA 297 



hard ground practically all of it ; the lateral digits (2d and 

 4th) which in existing horses are represented by the rudimen- 

 tary metapodials, or "splints," though much more slender 

 than the median digit, yet had the complete number of parts 

 and each carried a small hoof. Mere "dew-claws" as these 

 lateral toes were, they may have been of service in helping to 

 support the weight in mud or snow. In all parts of the skele- 

 ton there are little details which show that these species of the 

 middle Pliocene were not so advanced and differentiated as are 

 their modern successors, but it would be. unprofitable to enumer- 

 ate these details, which are of interest only to the anatomist. 

 In the lower Pliocene the horses were very much more 

 numerous and varied than in the middle portion of the epoch. 

 The same three genera of grazing animals, represented by less 

 advanced and modernized species, are found ; and, in addition, 

 there was an interesting survival (^Merychippus) frorn the 

 middle Miocene of an intermediate type, together with several 

 species of browsing horses {^Parahip-pus and \Hypohippus) . 

 In these browsing forms the teeth were all low-crowned and 

 early formed their roots, and the crowns were either without 

 cement or with merely a thin film of it in the depressions of the 

 grinding surface. The pattern of the grinding surface is so 

 very much simpler than in the high-crowned, prismatic teeth 

 of the grazers that it requires close analysis to detect the 

 fundamental identity of plan. Such teeth imply that their 

 possessors must have fed habitually upon a softer and less 

 abrasive diet than grass, probably the leaves and soft shoots 

 of trees and bushes and other succulent vegetable substances, 

 very much in the fashion of existing deer, and must therefore 

 have been chiefly inhabitants of the woods and groves and 

 thickets along streams, as the grazing species were of the 

 plains and open spaces. "This assemblage of the progressive 

 and conservative types of horses was certainly one of the most 

 distinctive features of Lower PUocene time in North America " 

 (Osborn). 



