MANUAL OF ZOOLOGY. 



indigenous. Closely allied to the true Rats are the Hamsters 

 ( Cricetu's, fig. 440), and the Voles (Arvicola} ; the latter repre- 

 sented by many species in both Europe and America. The 

 molars of the Voles (fig. 439) are composed of alternating tri- 



Fig. 439. Molar teeth of the Water-rat (Arvicola amphibius], 



angular prisms. Three species of Arvicola (the Water-rat, the 

 Field-vole, and the Black Vole) are found in Britain. 



A less familiar example of this family is the Lemming (Myodes 

 lemmus). This curious little Rodent is found inhabiting the 

 mountainous regions of Norway and Sweden. It is chiefly re- 



Fig. 440. Common Hamster (Cricetus vulgaris). 



markable for migrating at certain periods, generally towards the 

 approach of winter, in immense multitudes and in a straight 

 line, apparently in obedience to some blind mechanical im- 

 pulse. In these journeys the Lemmings march in parallel 

 columns, and nothing will induce them to deviate from the 

 straight line of march, the migration always terminating in the 



