122 O. C. Marsh—New Tertiary Mammals. 
Art. XXI1.— Preliminary Description of New Tertiary 
Mammals; by O. C. Marsa. Part L 
THE explorations of the Yale College party in the Rocky 
Mountain region, during the past season, brought to light, m 
addition to the extinct Birds and Reptiles already described by 
the writer, many interesting species of new fossil Mammals, 
and in the present communication a number of these from 
Wyoming Territory are briefly characterized. Others will be 
noticed in the succeeding numbers of this Journal, and it is 
intended, at an early day, to give a full description with illus- 
trations of all the new fossil vertebrates discovered by the two 
Yale expeditions of 1870 and 1871. 
Paleosyops laticeps, sp. nov. 
determine with certainty its exact specific relations. 
One of the treasures obtained by the Yale expedition of 1870, 
which first explored the Green River Tertiary basin, was the 
nearly complete skeleton of a species of Paleosyops, somewhat 
smaller than the one described by Dr. Leidy. The animal was 
adult, with the dental series in full perfection, although the 
epiphyses were not completely codssified with the vertebre. 
The teeth in this specimen have apparently the same general 
structure as those in the type of P. paludosus, but differ in beg 
nearly smooth; and this is not the result of age, as this individ- 
ual was younger than the original of the larger species. € 
ap ea moreover, given for the molar described* (‘22 
ines fore-and-aft and 18 transversely”), would not apply to any 
of the series in the present specimen. The last upper molar of 
the latter has two well developed internal cones. 
The cranium in Paleosyops laticeps is broad, and the zygomatic 
arches much expanded. The squamosal portion is especially 
massive. The nasals are narrow and elongated, and more like 
the corresponding bones in Ayrax than those in the larger 
pachyderms, They are prominently convex transversely, an¢ 
strongly arched longitudinally. The inner edges are thickened 
* Proceedings Philadelphia Academy, 1870, p. 113. ‘ 
