1883.] ., The Extinct Rodentia af North America. 45 
After a general view. of the species and genera, some deduc- 
tions as to the course of evolution of the order will be presented. 
Eocene RODENTIA. 
PLESIARCTOMYs Bravard. 
This is the prevalent genus ofi Rodentia ofi the Eocene period 
in- North America, Specimens were _ first. discovered, by Dr, 
Hayden in Bridger, beds of Wyoming, and were described by 
Dr. Leidy; I subsequently detected them in the Wasatch forma- 
tion of New Mexico: ` Their remains are rather abundant in both 
formations, but display but little variety of form. 
The teeth have short crowns and long roots, and have the gen- 
eral characters as well as numbers as the existing species of squir- 
rels.. There are, however, cranial characters which distinguish it 
from the existing forms of that family. The crowns of the infe- 
Fic. «Parts of twocrania and the ulna and radius of e ie delicatissi- 
mus pede natural Bite, from a-block of the Wasatch bed of the Big Horn river, 
Wyoming. Original, from Vol. 1v, Report U. S. Geol. Surv. ‘Senden 
_ ior molars support four rather small and strictly TREG tuber- 
cles. There are five superior molars, of which the anterior is of 
nal! size. They resemble those of Sciurus, but the transverse 
