IX.] RODENTS. 313 



being found throughout the forest regions of northern Europe, 

 Asia, and North America, while in the Plistocene era it ranged as 

 far south as England. Although marine mammals are for the 

 most part omitted in this work, the walruses (Trichechidcz), which 

 now have a circumpolar distinction, can scarcely be omitted, since 

 these animals never wander far from land. Remains of the 

 existing forms have been disinterred from the peat of the English 

 fens, while tusks of fossil species have been discovered in the 

 Pliocene Crag of the east of England, and also in the corre- 

 sponding deposits of Belgium. 



Passing on to the rodents, we have in the Sciuridce the ground- 

 squirrels or chipmunks ( Tamias] practically confined to the region. 

 Although in America they also extend into the Sonoran, in Europe 

 they are unknown in the Mediterranean area. The pouches in 

 the cheeks for storing food and the alternate dark and light longi- 

 tudinal bands down the back serve to distinguish the chipmunks 

 from other squirrels. Fossil remains of the genus, probably 

 belonging to existing species, occur in the Plistocene of Europe 

 and North America. The family of the beavers (Castoridcz) seems 

 always to have been mainly confined to the Holarctic region, one 

 of the two existing species ( Castor fiber] being European, and 

 dating from the English Plistocene, while the other (C. canadensis] 

 is North American. Whereas, however, the latter ranges south- 

 wards into the Sonoran region, the former is unknown in the 

 Mediterranean sub-region. Fossil species of this genus occur in 

 the upper Tertiaries of Europe and North America, where the 

 extinct Chalicomys is likewise met with ; but no beavers are known 

 from either the Siwaliks or the Pikermi beds. Although there are 

 few generic types of Muridce peculiar to the whole region, such as 

 there are are important. Foremost of these are the great tribe of 

 the voles (Microtus 1 }, constituting the typical representatives of a 

 sub-family nearly allied to the cricetines, but distinguished by the 

 two longitudinal rows of tubercles on the crowns of the molar teeth 

 being modified into alternating triangular prisms, and likewise by 

 these teeth being generally of the hypsodont type. In the Old 

 World they are found from the Arctic zone to Asia Minor, while 



1 Commonly known by the later name of Arvicola. 



