.60), narrower first upper molariform tooth (C. L. .75), and shorter distance between the alveoli of the first upper molars (C. L. .88). _Remarks._--The species, _Microtus pennsylvanicus_, in Pleistocene time ranged onto the plains of Kansas as far southward as Meade County, Kansas (Hibbard, 1940:421). This occurrence indicates a cooler more humid climate then than now in southwestern Kansas. _M. p. finitus_ is more closely associated with water than _Microtus ochrogaster_, the only other species of _Microtus_ now occupying the same region, although both species have been captured at certain places in the same runways. In Nebraska, a marginal part of the range of the species, _M. pennsylvanicus_ has been taken at scattered localities. This scattered and localized distribution of suitable habitat undoubtedly limits gene-flow between these relict populations. Presumably as a result of this isolation _finitus_ has accumulated and maintained its distinctive characteristics. The subspecies is so named because of its limited range. The average weight of eight specimens (4 males and 4 non-pregnant females) from Dundy County is 57.2 grams. _Specimens examined._--Total 26. _Colorado_: YUMA CO.: Wray, 3 (USBS); _1 mi. W Laird_, 2. _Nebraska_: DUNDY CO.: 5 mi. N, 2 mi. W Parks (Rock Creek State Fish Hatchery), 19; Haigler, 2 (USBS). Microtus pennsylvanicus pullatus new subspecies _Type._--Adult male, skin and skull, number 37873, University of Kansas, Museum of Natural History, obtained by Rollin H. Baker, original number 1343, 12 miles north and 2 miles east of Sage, 6100 ft., in Lincoln County, Wyoming, on July 19, 1950. _Range._--North-central Utah, eastern Idaho, western Wyoming, and southwestern Montana. See Figures 1 and 2. _Diagnosis._--Size average; color dark, especially in southern part of range; tail relatively long; molar teeth small; nasals relatively broad; maxillary septum relatively small.