MAMMALS 315 



nests under bush heaps or stones, to which runways, often more or less 

 subterraneous, lead; they feed on grasses, roots and bark, and also at 

 times on grain, and they do not hibernate. Field mice at times become 

 excessively numerous in a district, when they may destroy forage crops, 

 and also fruit trees and shrubbery by gnawing o& the bark at the base 

 of the trunk. Several litters, numbering from 4 to 11 each, are born 

 annually. 



Key to the United States Species of Micro tus 



El Plantar tubercles 6. 

 bi Mammae 8. 



Ci In the eastern and central States. 



di In the States east of the Mississippi. 

 Ci Nose not yellow. 



fi Present from the Atlantic to the Rockies. . .M. pennsylvanicus. 

 iz On Muskeget Island, Massachusetts M. brewer i. 



62 Nose conspicuously yellow M. chrotorrhiniis. 



d2 In the States between the Mississippi and the 



Rockies. 



Ci Present from the Atlantic to the Rockies M. pennsylvanicus. 



e2 In the Black Hills, South Dakota M. longicaudus. 



63 In the Rocky Mountains M. mordax. 



C2 In the Pacific States. 



di Throughout California M. calif ornicus. 



d2 In the San Joaquin and Sacramento Valleys M. edax. 



ds In the southern Sierras M. dutcheri. 



di In Idaho and Nevada M. nanus. 



ds In Oregon and Washington. 



Ci From Puget Sound to Yaquina Bay M. townsendi. 



• Co In the Willamette Valley M. canicaudus. 



Cs In northern California and southern Oregon.. .M. angusticeps. 



e4 In eastern Oregon in Utah M. montanus. 



Cs In the Olympic Mountains M. macriiriis. 



hi Mammae 4; in Arizona M. mogollonensis. 



a2 Plantar tubercles 5. 

 bi Mammae 8. 



Ci In the Boreal zone of the Rockies and Cascades M. richardsoni. 



C2 In the Puget Sound region M. oregoni. 



M. serpens. 

 b2 Mammae 6. 



Ci In the central Mississippi Valley M. ochrogaster. 



C2 From South Dakota and Kansas to the Rockies M. haydeni. 



C3 In Louisiana M. ludoviciamcs. 



Ci In Minnesota and South Dakota M. minor. 



