THE FORERUNNERS OF THE HORSE 273 



certain Lower Eocene species which have been 

 referred to the three genera Protorohippus, Eohippus, 

 and Hyracotherium, of which the last (also known 

 as Plioiophus] is European, while the other two 

 are American. Whether they may not all be 

 included in Hyracotherium is a matter of individual 

 opinion. The species described as Protorohippus 

 ventricolus, from the Wind River Eocene, was 

 somewhat larger than Eohippus pernix, standing 

 about 3^ hands (14 inches) at the shoulder, and 

 having rather longer limbs, in the front pair of which 

 the vestige of the thumb, or first toe, seems to have 

 disappeared, while the shortening of the fifth, or 

 outermost, toe is another step in the direction of a 

 three-toed foot. In the upper cheek-teeth there is 

 fuller development of the cross-crests, and while 

 the fourth premolar is molar-like, the third is 

 partially so. Hitherto remains of the genus have 

 been found only in Colorado and Wyoming. 



Eohippus, as represented by E. pernix, of the 

 Wasatch Eocene of North America, has been so 

 well described by Dr. Lull, 1 that his words may be 

 quoted in extenso. After observing that the upper 

 cheek-teeth are very similar to th6se of the allied 

 European genus, he goes on to say that they 

 display a sign of advance " in that the cross-crests 

 are somewhat more distinct than in Hyracother- 

 , and, unlike the latter, the fourth premolar is 



1 Op. tit., p. 1 7 1. 



S 



