3o6 VERTEBRATE ANIMALS OF THE UNITED STATES 



mostly northerly in range; both diurnal and nocturnal animals which 

 build 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 off the 

 bark at the base of the trunk. Several litters, numbering from 4 to ii 

 each, are born annually. 



Key to the United States Species of Microtus 



ai Plantar tubercles 6. 

 bi Mammae 8. 



Ci In the eastern and central States. 



di In the States east of the Mississippi, 

 ei Nose not yellow. 



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



fo On Muskeget Island Massachusetts M. breweri. 



e2 Nose conspicuously yellow M. chrotorrhinus. 



d2 In the States between the ^Mississippi and the 

 Rockies. 



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



to 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 southern Sierras M. dutcheri. 



ds In Idaho and Nevada M. nanus. 



di In Oregon and Washington. 



ei From Puget Sound to Yaquina Bay M. townsendii. 



62 In the Willamette Valley M. canicaudiis. 



63 In northern California and southern Oregon, . . M. angusticeps. 



64 In eastern Oregon in Utah M. montamis. 



b2 Mammae 4; in Arizona M. mogollonensis. 



a.1 Plantar tubercles 5. 

 b] IMammae 8. 



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



C2 In the Puget Sound region M . oregoni. 



M. serpens, 

 hi 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. Indovicianus. 



C4 In Minnesota and South Dakota M. minor. 



M. pennsylvanicus (Ord). Common held or meadow mouse (Fig. 

 170). Color chestnut brown above, darker middorsally, grayish 



