270 THE HORSE AND ITS RELATIVES 



as the EquidcE, but, as already mentioned, this is 

 not a satisfactory arrangement ; and the genus is 

 accordingly regarded here as the type of a distinct 

 family — the tiyracotheriidce. 



The members of this family are small and primi- 

 tive perissodactyle ungulates characterised by the 

 presence of four front and three hind toes, low- 

 crowned cheek-teeth, of which the upper molars 

 are usually much more complicated than the pre- 

 molars, and carry two oblique transverse crests, 

 each partially divided into two cusps, and connected 

 together by a longitudinal outer wall, a complete 

 ulna and fibula in the limbs, and the socket of 

 the eye entirely open behind. In place of the 

 cannon-bone of the horse, the basal portion of the 

 hind-foot is formed by three metatarsal bones, of 

 which the lateral pair are not smaller or shorter 

 than the middle one ; a similar structure ob- 

 taining in the case of the three main digits of 

 the fore-foot, where the basal bones are known 

 as metacarpals. In both limbs the lateral toes 

 touched the ground, the whole foot being thus 

 adapted for walking on the borders of swamps or 

 marshes or the muddy or sandy shores of lakes. 



From the simple upper molar o{ Hyracotherium, 

 with practically six columns on the crown, which 

 coalesce into a pair of cross-crests and an outer 

 wall, is derived by imperceptible gradations the tall 

 and complex molar of the horse ; the evolution 



