690 EVOLUTION OF THE HORSE. 



and 4th premolars resembled molars. The canines were 

 a little stronger than the incisors, and were separated 

 from the back teeth by a wide diastema (interdental 

 space). 



Mesohippus bairdii (Fig. 643). — " Near the base 

 of the Miocene, we find a third closely allied genus, 

 Mesohippus, which is about as large as a sheep, and 

 one stage nearer the horse. There are only three 

 toes and a rudimentary splint on the fore feet, and 

 three toes behind. Two of the premolar teeth are 

 quite like the molars. The ulna is no longer distinct 

 or the fibula entire, and the other characters show clearly 

 that the transition is advancing " (Marsh). The " rudi- 

 mentary splint on the fore feet " was a vestige of the 

 5th digit. The limbs had increased in length, and in 

 some cases were comparatively longer than in recent 

 horses ; the animal inhabited more open ground than its 

 ancestors, and, to judge by its teeth, its food was harder 

 and dryer than theirs. The canine teeth were still com- 

 paratively large, and the corner incisors were cupped, and 

 were beginning to show the " mark." The second pre- 

 molar was frequently like a molar. 



Mesohippus intermedius. — In this species, the 

 middle incisor was slightly cupped. The vestige of the 

 5th digit on the fore feet was smaller than in M. bairdii, 

 and the vestigial (2nd and 4th) digits varied considerably 

 in strength, which fact implies that they were no longer 

 of much importance in the struggle for existence. 



Miohippus (Anchitherium prestans, Fig. 643), is 

 found in the Upper Miocene of America, and was 

 about the size of a donkey. It had a very small vestige 

 of the 5th digit in front, and the vestigial (2nd and 

 4th) digits of both fore and hind limbs were slender. 

 The middle incisor was well cupped. The depressions in 



