268 
PERISSODACTYLA. 
last two premolars. In size, this species is less than the H. nanus^ the 
smallest of the genus. The third and fourth premolars have two transverse 
crests each, of which the anterior forms the border of the crown, and the 
posterior marks its middle. The supernumerary cusp at the external anterior 
angle of the crown is well marked in both premolars and molars; the pos¬ 
terior transverse crest of the molars is less elevated than the anterior as it 
approaches the longitudinal cusps. The latter are well joined together, and 
are strengthened by an external vertical rib, which extends to the apex. 
There is a weak cingulum on the anterior base of the crown, which passes 
round the inner base of the anterior internal cone, but is wanting at the base 
m 
of the posterior, and very weak on the posterior and exterior bases of the 
crown. Enamel smooth in young as in old teeth. 
Measurements. 
M. 
Length of five posterior molars. 0.0330 
Length of the third premolar.... 0 0054 
Width of the same. 0. 0056 
Length of the second true molar... 0. 0080 
Width of the same... 0.0100 
Length of three true molars..... 0. 0220 
This species is, as observed in my preliminary report, the only repre¬ 
sentative of the numerous Hyracliyi of the beds of the Bridger formation. 
In my exploration of this horizon in Wyoming, I obtained remains of six 
species in great abundance, so that the existence of a single small species in 
few individuals, constitutes a marked feature of the New Mexican Eocene 
fauna. The absence of Palceosyops^ a genus perhaps still more abundant 
than HyracJiyus, in Wyoming, adds to the evidence in favor of the belief 
that the difference between the faunae of the respective localities is due to 
something more than the peculiarity of geographical distribution, but points 
to diversity of horizon or time, as is, indeed, sufficiently indicated by the 
study of the stratigraphy in Wyoming. There the Green Kiver beds con¬ 
tain the same fauna as those under consideration, and underlie the Bridger 
formation. 
