Article XIII.— A NEW THREE-TOED HORSE. 

 By J. W. GiDLEY. 



The Expedition of 1902, sent out by Prof. Henry F. Osborn 

 to explore the Miocene exposures in South Dakota, had tlie 

 good fortune to obtain, besides other material, a complete 

 skeleton of a large three-toed horse (Amer. Mus. Coll. No. 

 9815), associated with incomplete skeletons of five other in- 

 dividuals, undoubtedly of the same species. 



This splendid specimen, which represents an undescribed 

 genus and species, was discovered by Mr. H. F. Wells, a mem- 

 ber of the party, in the upper Miocene deposits on Little 

 White River, near Rosebud Agency, South Dakota. 



The characters presented, especially in the teeth, if inter- 

 preted according to former authors, would undoubtedly 

 place the present species in the genus Hippariuii. However, 

 as indicated by a careful study of this new material, and of 

 the abundant material of other Miocene horses in the Ameri- 

 can Museum collection, together with a comparison with 

 specimens and descriptions of the European forms, it seems 

 probable that the genus Hipparioii is limited in distribution 

 entirely to the Old World, and that the American species for- 

 merly referred to this genus should be placed in a group dis- 

 tinct from Hipparion. 



Before describing the new skeleton, therefore, the writer 

 wishes to point out the chief characters which distinguish the 

 Old World from the New World forms. 



The characters common to both groups are as follows: 

 (i) Column of protocone of the upper molariform teeth en- 

 tirely surrounded by cement; (2) the lower molars of the 

 milk dentition possess a median external basal tubercle; (3) 

 each foot possesses three complete toes, the lateral ones being 

 much reduced. 



The characters which especially distinguish the true Hip- 

 parion a.re: (i) Protocone cylindric or subcylindric through- 

 out the greater part of its length. (2) Enamel borders of the 

 [yuly, iQOj.'] 465 ^^ 



