ORDER UNGULATA. 



1361 



form having been made the type of the genus Hippodactyhts. The 

 best known species is H. gracile of the Pliocene of Europe, Samos, 

 Persia, and Algeria ; but the genus is also well represented in the 

 Pliocene of India (H. a?itilopinum, H. Theobaldi), China (H. Richt- 

 hofeni\ and North America. All the species retain the primitive 

 feature of a depression in the lachrymal. With Equus (in which 

 may be included the American Hippidium, otherwise Pliohippus) 

 we come to the most specialised of all the Perissodactyla. The 



dentition is I. 



C. 



I Pm. ^ 4 ( , M. - ; but the first upper cheek- 

 1 (3-4) 



3 1 {3-4) 3 



tooth is usually absent in existing forms, and the corresponding 

 lower one is only occasionally developed in some extinct species. 

 The crowns of the cheek-teeth are higher than in Hipparion, and 

 the anterior inner pillar of the upper ones, except in a very early 

 stage of wear, is connected with the adjacent inner crescent (fig. 

 1234). There is but one functional digit to each foot, although the 



Fig. 1234. — The last four right upper cheek-teeth of the Horse {Equus caballus). 

 Reduced. 



proximal portions of the lateral metapodials remain (figs. 1232, 

 1236, d), and in the so-called Hippidium the terminal phalangeals 

 were represented by claws. A maxillo-lachrymal fossa is present in 

 the extinct E. andium and E. siva/ensis, but is wanting in all exist- 

 ing species. At the present day this genus is confined to the Old 

 World, and is especially characteristic of Africa, but in the Pliocene 

 and Pleistocene it was spread over both North and South America. 



In those South American Pleistocene forms referred by some writers 

 to Hippidium, the molars are shorter and more curved than in exi sting- 

 species, and the grinding surface of the anterior pillar of the upper ones 

 is not wider than in Hipparion ; E. principalis is a large species 

 of this type. In E. Stenonis of the Upper Pliocene of Italy, Kos, and 

 Algeria, and the Norfolk Forest-bed, the molars are taller, but they still 



