92 Bulletin American Museum of Natural History. [Vol. XIV, 



distributed, and very abundant in some sections, it was not until 

 the discovery by the writer in the summer of 1899 of the excellent 

 material upon which he founded the species Equus scotti that 

 anything like a complete skull had ever been found in the United 

 States; nor had the teeth and skeleton of a single individual ever 

 been found certainly associated. The materials upon which the 

 North American species were founded, and which formed the 

 basis of the descriptions and discussions of Owen, Leidy, and 

 Cope, were very fragmentary, for the most part consisting of dis- 

 associated teeth and bones, among which portions of maxillary 

 bones containing several teeth were rare specimens; hence a 

 number of the species proposed were founded on single disasso- 

 ciated teeth. Owing to this lack of good material and to an im- 

 perfect understanding of the characters presented in the teeth, 

 errors have been frequent and several of the species of this genus, 

 as they now stand, are practically indeterminate. 



This is the conclusion reached by the writer after a care- 

 ful study of the abundant Equus material from the Niobrara 

 River (collected by the American Museum expeditions of 

 1893 and 1897 '), together with the skulls and associated partial 

 skeletons from the Staked Plains of Texas, and of numerous 

 specimens of the living species. 



The object of the present paper is to present the results of this 

 study and to attempt a revision of the species that have been 

 proposed. 



Before proceeding with the discussion, however, the writer 

 wishes to extend his thanks to Prof. Henry F. Osborn for his 

 valuable advice and aid in the systematic arrangement of this 

 paper; to Mr. F. A. Lucas of the National Museum and Dr. C. R. 

 Eastman of the Museum of Comparative Zoology, Cambridge, 

 Mass., for making possible the examination of some type speci- 

 mens and associated material; to Mr. Witmer Stone of the 

 Academy of Sciences, Philadelphia, for assistance in examining 

 the Equus material in the Academy that was used or identified 

 by Dr. Joseph Leidy and Prof. E. D. Cope; and to Dr. W. D. 

 Matthew and Dr. O. P. Hay of the American Museum, for valu- 

 able suggestions and assistance. The drawings for this paper 



• This material consists of a large number of mostly disassociated bones, teeth, jaws, and 

 skull fragments. There are a few complete and several nearly complete molar-premolar series 

 in the lot. 



