MAMMALS OF MINNESOTA. 253 
- The mice appear in both Europe and America in the Eocene, 
some naturalists identifying the genus Arvicola in South Ameri- 
can strata of that age. In the Miocene many additional forms 
appear as, in Europe, the jumping mouse (Jssyodromys), the 
beaver, and beaver-like animals( Chalicomys and Steneofiber). The 
Miocene was the time when* the rodents expanded to their pres- 
ent proportions, since which time few changes of importance 
have taken place. The progenitor of the Rodentia is as yet 
undiscovered, and speculation will have little weight until some 
truly intermediate forms are discovered. The oldest remains 
thus far discovered are truly rodent-like. 
