258 THE HORSE AND ITS RELATIVES 



molar is the great complexity of the foldings of 

 the enamel in the central islands of the crown. 



The North American hipparions, which occur 

 in the Miocene, have been separated generically 

 from their Old World relatives by Mr. J. W. Gidley' 

 as Neohipparion. From the typical hipparions 

 the American species are distinguished by the 

 larger size and more elliptical form of the grinding 

 surface of the anterior pillar of the upper cheek- 

 teeth ; the simpler folding of the enamel of their 

 central islands ; and the concave external walls of 



Crown Surface of Left Upper Molar of the Extinct American Hipparion, 

 \ natural size, pr, anterior pillar, or protocone 



their outer columns. The limbs, more especially 

 the cannon-bones, are also of a longer and more 

 slender type, and the lateral toes appear to be 

 relatively small. Finally, they antedate the Old 

 World species in time, and may thus be near akin to 

 the ancestral form of the latter, if indeed they be not 

 the actual ancestors. At most, however, they are 

 only worthy of subgeneric distinction. 



In regard to the anterior pillar, or protocone, 

 of the American genus, Prof. Lull' remarks that 



^ Bull. Atner. Mus. Nat. Hist., vol. xix. p. 465, 1903. 



* Op. cit., p 179- • 



