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 (Issyodromys), the 

 beaver, and beaver-like animals ( Chalicomys and Steneo/iber ) . 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. 



