THE FORERUNNERS OF THE HORSE 251 



by Prof. Osborn.' That the ancestors of the two 

 South American genera came from the north is 

 practically certain ; and it is therefore probable 

 that such ancestors were identical with or closely 

 allied to either Pliohippus or Protohippus. 



Hippidiuiu as typified by H. neogcsum and its 

 near relative Onohippidium mtcnizi\^QY& small, heavy- 

 headed horses, differing in many important details 

 from all other members of the family. In both 

 genera the cheek-teeth have shorter crowns and 

 differ in several details of structure from those of 

 modern horses. A cast of the skeleton (pi. xxiii.) 

 in the British Museum stands i2\ hands at the 

 withers, while the skull measures 23^^ in. in total 

 length. In a European horse-skeleton standing 14-^ 

 hands the skull-length is about 23f in., or practi- 

 cally the same as in the much smaller Hippidimn. 

 Comparison of the skull of the latter with that 

 of an ordinary horse shows a remarkable differ- 

 ence in the structure of the nasal region. In 

 the horse the nasal bones are separated from the 

 maxillae, or upper jawbones, of each side by a slit 

 of only some three or four inches in length. 

 In Hippidium and the allied Onohippidium (pi. xxiv. 

 fig. i), on the other hand, these slits are about 

 \o\ in. long, while the nasal bones themselves are 

 proportionately long and slender. This indicates 

 that these extinct American genera had extremely 



* Op. cit., p. 356. 



