272 THE HORSE AND ITS RELATIVES 



seems to be closely allied to Epihippus, having 

 upper premolars of the same complex type. 



The Middle, or Bridger, Eocene of North 

 America has yielded remains of animals referred to 

 two distinct genera, namely Orohippus and Helio- 

 hippus, of which the latter is known only by the 

 teeth. Whether these are truly distinct, or whether 

 they are really separable from the contemporary 

 European Pachynolop/ms, which was named at a 

 much earlier date, may, however, be a matter of 

 opinion. In justification of this assertion it may 

 be well to mention that the limitations of genera 

 and larger groups of animals are purely arbitrary, 

 and therefore dependent in great measure on in- 

 dividual opinion.^ 



Oi^ohippus includes several species, such as 

 O. agilis and O. majoi'. In structure these show a 

 slight advance on the undermentioned Prot orohippus, 

 and are likewise slightly superior in size. The 

 foot-structure is much the same in the two, but 

 the cheek-teeth are rather more complex, the third 

 and fourth upper premolars being molar-like, and 

 the second approximating to the same type. The 

 gap between the front and the cheek-teeth is also 

 somewhat longer. 



The last group of the family is represented by 



^ See an article by Mr. L. Clark, " On the Purpose and some 

 Principles of Systematic Zoology," in the Popular Science Monthly, 

 September 191 1. 



