'22 .l.U/v'A'/r.l.V MCSh'CM criDh: LEM'LKTS 



than ill tlic earlier stages and are a|)j)ai-eiitly useless, as ihev cannot reach 

 the jijround. In some species they have almost disappeared. J; The fore 

 feet of tlies(» rpj)er Miocene horses still retain the tiny nodule of l)one at 

 the l)ack of the " knee" (the joint that corresj)onds to the wrist of a man), 

 which is tile last reniiiant of the fifth di^it j)()ssessed hy their remote 

 ancestors. 



Ilippariofi is coniinon in Imiiojx' and Asia as well as in North 

 America, hut Protohippus and Pliohippiis ai'c not found in the ()l(l 

 World. 



Fig. 12. Hippnriori. Lower Pliocene. I'pper teeth one-half natural .size. 

 Long-crowned teeth, heavily cemented 



ONE-TOED HORSES 



12. Pliohippus. Pliocene. In some species of Pliohippus, 

 although not in all, the side toes are still further reduced to long slender 

 sphnt-like bones which are enclosed wdthin the skin, the little separate 

 toes havinjj; disappeared. These are the first of the One-toed Horses. 

 The splint is nearly as long; as the cannon bone, much longjer thaji it is 

 in Equu,s. Two fine skeletons of P/?o/??'pp?/s in the Museum collections 

 (P. leidyanus, adult, and P. luUianus, 3'oung) acbnirably illustrate this 

 stage.^ The}' are of somew^hat later geologic age than the Hipparion 

 skeleton, and nearer to the modern horse in size, proportions and 

 various details of structure. Yet the}' are so much like other sj)ecies of 

 Pliohippus in which the side toes are still complete that both are placed 

 in the same genus. Pliohippus is therefore transitional from the three- 

 toed to the one-toed stage. It may ])e that some of the later sjxH'ies of 

 Hipparion were also one-toed, but at present thei'e is no j^roof of it. 



13fl. Equus. Pleistocene and Recent. In this stage, that of the 

 modern horse, the side toes have entirely di.^appeared and are represented 

 by moderately long splints on the fore and hind foot. No trace remains 

 on the fore foot of the little nodule which in previous stages represented 

 the fifth digit, while on the innei- side of the wrist the "trapezium," 



^IHi()hi)tpiis wa.s <loscril)(Mi in 1S74 by I'rofcssior Marsh, as a one-topcl ancestor of the horse; but his 

 specimens do not prove that the side toes were certainly absent. These two skeletons found in 1917 by 

 }i. J. Cook and 11)10 by E. H.Troxelishow ihis conclusively, and we are therefore able to distinguish 

 them a.s a separate stage in the evolution of the horse. In previous editions of this guide, 1903, 1905. 

 19i;^ this stage was not distinguished, as conclusive proof from specimens had not been found. 



