174 R. S. Lull — The Evolution of the Horse Family. 



though the differences between them are slight. Mesohippus 

 from the White River beds, the sixth stage, is three-toed in 

 both fore and hind feet, with a rather long splint bone repre- 

 senting the iifth digit of the hand, while in the foot there is 

 no trace of the outer one remaining. The entire series of 

 cheek teeth except the small first premolar is molariform, 



t-v 



Fig. 9. a, Fore foot and b, hind foot of Mesohippus celer. One-half 

 natural size. (After Marsh.) 



while the crests are not only well developed but there is a 

 tendency toward a further complication of the grinding sur- 

 face. 



Mesohijypus baircli, the best known form, averaged about 

 eighteen inches in height, a slender-limbed creature, very well 

 adapted for speed. Mesohippus intermedins was much larger 

 and in some ways un progressive, which, together with the con- 



