848 /'-'. /.. Troxell — Early Pliocene One-Toed Horse. 



and teeth render it of unusual value for comparative study. 

 I\ plaoidus is usually found in the Miocene, and this small 

 horse, without the malar pit and with semi-functional lateral 

 toes, shows characters seemingly too primitive for the Pliocene 

 period. 



The fauna of this Oak Creek formation corresponds closely 

 with that of the Snake Creek Beds of Western Nebraska; the 

 latter, though resembling the Republican River Beds of West- 

 ern Kansas, show a more modernized type of animal life and 

 are considered by Matthew and Cook to be intermediate 

 between the Blanco of Texas and the Upper Miocene. The 

 Oak Creek formation, while in some respects like the Etelie- 

 goin of California, Middle Pliocene, is not so far advanced 

 and in all probability belongs to the Early Pliocene. 



IV. General Conclusions. 



Plio/dppus hdlianus, the earliest one-toed horse now known, 

 is here made the type of a new species. It is tentatively 

 assigned to Pliohippus Marsh, awaiting the final settlement 

 of the status of that genus, which, founded upon an imperfect 

 specimen, has been alternately accepted and rejected. 



Observations by Merriam, Osborn, Lull and others, point to 

 the protohippine horses as the group from which the modern 

 race was derived, and it is probable that Pliohippus lullianus 

 sp. nov. was near if not directly in the ancestral line. Through 

 its unique characters it seems to offer the connecting link 

 between its three-toed ancestors and the monodactylous Equus. 



The fauna indicates that the age of the beds, from which 

 the new type came, is Early Pliocene, a period of grass-covered 

 plains. Because the climate was semi-arid and there was little 

 stream action, the deposits of that period are rare and at the 

 present time are nearly always hidden beneath the luxuriant 

 vegetation of the region. 



