PERISSODAOTYLA. 265 
Measurements. 
M. 
Length of the last two naolars. 0. OKJO 
Length of the penultimate molar. .... 0.0085 
Width of the same. 0. 0100 
This species resembles the U. tapirimm in the robustness of the jaws, 
but the teeth are materially smaller. I originally discovered it in the 
Wasatch beds of Bear River and Black Butte, Wyoming; the other species 
from the former horizon {H. index) has not yet been detected elsewhere. 
Hyracotherium angustidens, Cope. 
Plate Ixvi, figs. 1-6. 
Orohippus angustidens. Cope, System. Cat. Vert. Eocene New Mexico, U. S. Geog. Survs. 
W. of 100th M., 1875, p. 22. 
A number of jaws were obtained which resemble in general propor¬ 
tions those of the Hyracotherium index, but differ in the greater length of 
the series of the premolars. These teeth are nearly as large as the corre¬ 
sponding ones in H. sylvaticum, but the other teeth and the ramus of the 
jaw belong evidently to a smaller species, and one near to the least forms 
of the genus. 
Selecting for description a portion of a ramus which supjiorts the last 
tliree molars, we observe the close resemblance between the latter and 
those of other species. The anterior ledge and posterior median tubercle 
are little developed; the oblique ridges are well developed, and the heel of 
the last molar elevated. The molars have a strong cingulum on the outer 
side, and the enamel is slightly rugose. The ramus is slender. 
Measurements. 
N ■ M. 
Lengtli of the bases of three true molars . 0. 024 
Length of the penultimate molar.... 0. 007 
Width of the same..... 0. 005 
Depth of the ramus at the last molar. .. 0. 0125 
In another specimen of about the same size, the bases of three premo¬ 
lars are preserved, and measure 0“.016 in length. In another, which sup¬ 
ports the crowns of the third and fourth premolars, the characters are well 
displayed. The third has a short cusp on the posterior heel, and a sharp 
cusp on the inner side of the principal one. There is a rudiment of an 
anterior basal cusp. The anterior cusps of the fourth premolar are equal. 
