48 The Extinct Rodentia of North America. [January, 
that its inner and outer plates are well developed, and sub- 
equal, 
The bones of the limbs are generally similar to those of the 
Sciuridz. In this family the genus Gymnoptychus nearly resem- 
bles Ischyromys in dental characters. But besides the important 
difference in the former and position of the infra-orbital foramen, 
-Ischyromys has an excavated posterior palatal border. 
Fic. 3.—Zschyromys pus Leidy, natural size, from the White river beds of Color- 
ado, original, from the Report U. S. Geol. Surv. Terrs. a; 4, c, cranium; g, mandi oe 
from above. 
Dr. Leidy remarks that this genus belongs to the family of the _ 
Sciuridz. This is indicated by the dental characters ; but in some — 
other respects there is a greater divergence from the squirrels and _ 
marmots than is the case with the genus Gymnoptychus. Thus, 
the large foranien infraorbitale anterius occupies the elevated 
position at the origin of the zygomatic arch seen in the porcupines 
and cavies. There is no superciliary ridge nor post-orbital pro- 7 
cess as in most Sciuridæ, but the front is contracted between the 
orbits in the same manner as, but to a less degree than, in Fiber, 
and the Eocene Plesiarctomys Brav. Both the last named and 
Ischyromys present many points of Aiet epa to Pomel’s tribe 
i 
