4 University of California Publications in Geology [Vol. 9 



Miocene. E. eurystylus is distinguished by extreme narrowness of 

 the lower molar crowns, by the great anteroposterior diameter of the 

 metaconid-metastylid column and its close ap- 

 pression to the protoconid and hypoconid, and 

 by the presence of an anteroexternal ridge on 

 the protoconid and one on the hypoconid. Those 

 characters of E. eurystylus appear in no. 19414 

 from Thousand Creek. The lower molar (M 2 ) 

 from Thousand Creek is very long and narrow 

 and was heavily cemented. The anteroexternal 

 ridges on the protoconid and hypoconid are 

 strongly developed, as in E. eurystylus. As in 

 Cope's species, the external ridge on the hypo- 

 conid forms a prominent ridge on the middle of 

 the external face of the crown. 



Gidley 3 held that Cope's Equus eurystylus 

 represented the genus Hipparion. While it is 

 true that the anteroexternal ridge of the proto- 

 conid and most of the other characters of E. 

 eurystylus may appear in milk teeth of Equus, 

 Cidley's reference of this form to a group in or 

 near Hipparion is well supported. 



Specimen no. 19414 from Thousand Creek 



3 



Fig. 3. N eohipparion 

 leptode, n. sp. M.,, no. 

 19414, type specimen, represents a species in many respects near Neo- 



natural size. loner and Upparion eury style? (Cope). The Thousand 

 occlusal views, Thousand 11 u -j \ i / 



Creek, Pliocene, Thou- Creek form is larger and the details of structure 

 sand Creek, Nevada. ,„ . ,, , . „, . . , ., 



are sufficiently different to separate the two, at 



least, tentatively. It also resembles other American forms of the 

 Hipparion group, as N eohipparion oecidentale, in many characters, but 

 seems not clearly referable to these species. The writer has, there- 

 fore, referred the Thousand Creek form tentatively to a distinct species. 



Measurements 



N. leptode, 



no. 19414 N. eurystyle 



M,, greatest anteroposterior diameter 29+ mm. 24 



M 2 , transverse diameter 11 10 



M,, greatest length of crown 66 55 



M 2 , anteroposterior diameter of metaconid-metastylid column 14.6 13 



s Gidley, J. W., Bull. Am. Mus. Nat. Hist., vol. 14, p. 126, 1901. 



