268 THE HORSE AND ITS RELATIVES 



circumstances that the splint representing the fifth 

 toe on the outer side of the fore-foot is smaller 

 in Miohippus (pi. i. fig. 2) than in Mesohippus^ 

 and also that the cheek-teeth are somewhat more 

 complex. The figure on p. 267 of an upper molar 

 of Anchitherium illustrates the elements which 

 go to form the constituents of the horse's molar. 



As will be inferred from the preceding para- 

 graph, Mesohippus differs from Anchitherium by 

 the presence of a rudiment of the outermost or fifth 

 digit of the fore-foot, while it is further distin- 

 guished by the absence, or at all events very 

 slight trace, of the pits in the crowns of the 

 incisors. Yet another feature is the presence on 

 the heel-bone, or calcaneum, of a small facet for 

 the articulation of the fibula, which is thus proved 

 to be complete. It should be added that in 

 Anchitheritim, Miohippus, and Mesohippus the 

 "wolf-tooth," that is, the first premolar, of the 

 horse is fully developed in both jaws, although 

 considerably smaller than the second premolar. 

 As canines were developed in both jaws, the full 

 typical series of 44 teeth was present, viz. : i. f , 

 c. X' /• 4» ^- f • '^^^ typical Mesohippus bairdi, 

 from the Oligocene of Dakota, was a slenderly 

 built animal, apparently well adapted for speed, 

 but standing only about 4^ hands (18 inches) at 

 the withers. M. hitermedius was a larger but 

 apparently unprogressive type. 



