E. L. Troxell — Early Pliocene One- Toed Horse. 335 



Art. XXXVI. — An Early Pliocene One-Toed Horse, Plio- 

 hippus lullianus, sp. nov. ; by Edward L. Troxell. 



Contents. 

 I Introduction. 



II Pliohippus lullianus sp. nov. 

 The teeth. 

 Skull. 



Preorbital pit. 



Ramus, atlas and axis. 

 Fore limb. 



Radius and ulna. 



Metacarpals. 



Third phalanx, hoof. 

 Measurements. 



III Geology of the Oak Creek formation. 



Character of the deposits. 

 Associated fauna. 

 Age of the beds. 



IV General Conclusions. 



I Introduction. 



Ninety years ago the first fossil horse was discovered in 

 America, but not until 1856, when Dr. J. W. Leidy described 

 the type of Protohippus perditus, did the presence of a race 

 of extinct horses especially attract the attention of anyone. 

 For the last half century the interest in these animals has been 

 steadily growing and in very recent years expeditions have 

 been sent out for the single purpose of securing their skeletons. 



The discovery last summer of a skeleton in the Early Plio- 

 cene, with but one toe on each foot, yet allied to the protohip- 

 pine horses which are known to be mostly tridactyl, furnishes 

 a new type of more than ordinary importance. The species is 

 described at this time as a matter of expediency, under the 

 genus Pliohippus Marsh, in order not to encroach upon 

 Professor Osborn's revision of the horses which is about to be 

 published. 



It gives me pleasure to name the new species in honor of 

 Professor Richard S. Lull of Yale. Pliohippus lullianus, 

 then, sp. nov. is the chief subject of this paper. 



The list of those who contributed to the success of the sum- 

 mer's expedition or who have aided in the preparation of this 

 article is even longer than can be recorded here. The Rev. A. 

 B. Clark of Rosebud took an interest in my work and directed 

 me to the region where the specimen was found. Professors 

 E. C. Case, H. H. Bartlett and Dr. F. E. Robbins, of Michigan 

 University, have examined my manuscript and their kindly 

 criticisms have been invaluable. Dr. W. D. Matthew and 



