116 



THE AGE OF MAMMALS 



uncertain relationships. The carnivore Arijillotherium (Davies, 1884) is 

 also indeterminate. 



Horses. — Fortunately the primitive four-toed horses (Equidse) are 

 represented by several highly characteristic specimens of the genus Hyra- 

 cotherium { = Pliolophus) . The great English anatomist Owen' descriljed 



these specimens {H. 

 leporinum and H. cuni- 

 culus), but quite natu- 

 rally failed to recognize 

 their ancestral relation- 

 ships to the horses. 

 The type {H. leporiniwi) 

 exhibits simple grinding 

 teeth (Fig. 2) which 

 are similar to those of 

 Eohippus borealis of the 

 Wasatch and Wind 

 River Formations of the 

 Rocky Mountains, but 

 the second superior premolar tooth is a very simple, two-rooted, single- 

 cusped tooth, whereas in all the American equines the same tooth is 

 more complex, namely, invariably three-rooted and three-cusped, or with 

 two external cusps and an internal ledge. This London Clay type of 



Fig. 33. — -Skull of the primitive Eocene horse Hyracothe- 

 rium (Pliolophus) vulpiceps of the London Clay (X ^). After 

 Owen. 



Fig. 34. — Models of the Lower Eocene, primitive horse of North America, Eohippus. 

 originals by Charles R. Knight in the American Museum of Natural History. 



After 



Hyracotherium, therefore, is the most primitive horse certainly known, 

 and bespeaks the very early entrance of the horses into Europe. H. vul- 

 piceps, or the 'fox-headed' hyracothere, also from the London Clay (see 



1 Owen, R., Trans. Geol. Sac, Vol. VI, 1839 (1841), p. 203, and Ann. Nat. Hist., Vol. VIII 

 (1841), 1842, p. 1. 



