HISTORY OF THE PERISSODACTYLA 



343 



attaining the proportions of the feet in the White River horses 

 {\Mesohippus). There were three digits in each foot, and the 

 median toe (third of the original five) was so much enlarged 

 and the lateral toes (second and fourth) so reduced, though 

 still functional, as strongly to suggest a monodactyl foot as the 

 outcome of this course of development, had not the early 

 extinction of the subfamily put an end to it. It is interesting 

 to reflect that, had the flophiodonts and fhyracodonts con- 

 tinued their existence to the present time and had persisted in 

 advancing along their particular Unes of specialization, we 

 should, in all probaBility, have had monodactyl tapirs and 

 rhinoceroses, as well as horses. 



As in the case of so many other mammalian series, the 

 jhyracodonts of the but partially explored Uinta formation 

 are still very imper- 

 fectly known. Al- 

 most all that can be 

 positively stated 

 about them is that 

 they were smaller 

 than their White 

 River successors and 

 that the assumption 

 of the molar-pattern 

 by the premolars was 

 incomplete. In the 

 upper Bridger beds 

 also not very much 

 is known regarding 

 the then representa- 

 tives of the series, 

 {'\Triplopus). So 

 much is clear, how- 

 ever, that they were still smaller and lighter animals, that 

 the limbs were very light, and that the number of digits in 



Fig. 181. — Left manus of t cursorial rhinoceroses. 

 + Triplopus cubitalis (after Cope), upper Bridger. 

 t Hyracodon nebrascensis. White River. 



A, 

 B, 



