262 



University of California Publications. 



[Geology 



feature of all of these specimens is seen in a small but distinctly 

 developed fold on the posterior side of the ridge of the hypoconid 

 extending toward the metastylid. 



This form is evidently the least common of the Virgin Valley 

 horses, Merychippus having been the most abundant, and Hypo- 

 hippus more common than Parahippus. 



Occurence : Virgin Valley Beds ; localities 1090 and 1095 ; 

 Virgin Valley, Humboldt County, Nevada. 



Measurements 



Inferior cheek tooth, P 4 ? 



Anteroposterior diameter of crown 20.5 mm. 



Greatest transverse diameter of crown 16.7 



Height of slightly worn crown, measured at metaconid 11.9 



MERYCHIPPUS, near ISONESUS (Cope) 



Teeth of Merychippus are the most common remains of fossil 

 horses in Virgin Valley, where they occur in association with 

 those of Hypohippus. They are found also at High Rock Canon, 

 farther to the south, but have not been seen in the beds at 

 Thousand Creek. 



In the collections obtained at Virgin Valley in 1906 Gidley 18 

 has recognized four forms of Merychippus teeth. These included 

 a form referred provisionally to Merychippus isonesus (Cope), a 

 second species (Gidley, species indet. 1) considered as possibly 

 representing a new form of Merychippus with Protohippus 

 affinities, a third (Gidley, species indet. 2) which was compared 

 with Merychippus seversus (Cope), and a fourth (Gidley, species 

 indet. 3) represented by a comparatively higher and straighter. 

 crowned form than the others. 



In the larger collections now available from Virgin Valley 

 the several forms present do not appear to represent any types 

 other than those referred to by Gidley. Unfortunately the 

 material nearly all consists of scattered teeth, excepting a few 

 fragments of lower jaws with teeth. As the lower teeth are not 

 associated with the upper dentition it is not possible to determine 

 with certainty their relation to the forms described by Gidley, 



is Gidley, J. W., Univ. Calif. Publ. Bull. Dept. Geol., vol. 5, p. 238, 1908. 



