466 Bulletin American Museum of Natural History. [Vol. XIX, 



fossettes of the upper molariform teeth very elaborately folded. 

 (3) The middle portion of the external walls of the meta- and 

 paracones is fiat or slightly convex. (4) The external median 

 tubercle in the lower milk molars is relatively high and circu- 

 lar in cross-section. 



The American group differs from Hipparion in the follow- 

 ing characters: (i) The protocone is relatively larger and 

 elliptical in cross-section, or with the outer wall flat to con- 

 cave. (2) The enamel foldings are in general comparatively 

 simple. (3) The external walls of the meta- and paracones 

 are concave. (4) The external median tubercle of the lower 

 milk molars is elliptical in cross-section and less prominent 

 than in Hipparion. (5) The limbs and feet, so far as known, 

 indicate a comparatively more slender construction of the 

 long bones and especially longer proportions of the meta- 

 podials. There is also apparently a relatively greater reduc- 

 tion of the lateral digits in the American genus. 



These characters, as stated above, have the more signifi- 

 cance from the fact that nearly all the American species are 

 Miocene, while those of the Old World are of Pliocene age. 

 It will be seen that in the development of the protocone and 

 the ectoloph in the upper teeth, and the proportions of the 

 feet and limbs, the Ainerican species, though coming from an 

 older formation, are more progressive than the Pliocene 

 species of Europe; while in some other respects, especially 

 the complicated foldings of enamel in the upper teeth, the 

 Old World species are more progressive. The reasons for 

 separating these two groups are further strengthened by the 

 fact that there are apparently no species common to both 

 hemispheres. 



From the foregoing it seems apparent that a new term is nec- 

 essary for the American species hitherto referred to Hipparion, 

 and they may be distinguished by the name Neohipparion. 



The following description is based on the complete skeleton 

 above referred to and is named in honor of Mr. William C. 

 Whitney, whose generosity made possible the expedition which 

 secured this very valuable acquisition to the present knowl- 

 edge of American fossil horses. 



