_Discussion._--This subspecies is separated from _M. m. codiensis_ to the west by the Big Horn Basin. A series from along Buffalo Creek, 27 mi. N, 1 mi. E Powder River, 6075 ft., in Natrona County, Wyoming, is intermediate between the topotypes of _zygomaticus_ and _nanus_ in the characters cited above as distinguishing the two, but shows greater resemblance to _zygomaticus_ in the shape of the zygomatic arch, in color which is paler than in topotypes of _zygomaticus_, and in the short hind foot. On these and on geographic grounds this population is referred to _zygomaticus_. Unfortunately we cannot be certain in many cases that an intermediate condition in a certain character indicates a genetically intermediate population and therefore true intergradation between the two subspecies to which the population is geographically intermediate. The topotypes of this subspecies are the most distinct of all the series which I have studied from the eastern Rocky Mountains, in terms of the degree of morphological departure from the norm for the species. After _zygomaticus_ the following populations are arranged according to their degree of deviation from this norm (_codiensis_ deviates most): topotypes of _codiensis_, _fusus_ and a population from southern Sweetwater County, Wyoming, and lastly the _nanus-caryi_ complex. Within the latter group, as I have mentioned, there are a number of local variants most of which do not differ significantly and do not conform to any geographic pattern. _Specimens examined._--Total, 55, distributed as follows (abbreviations for collections are given in the account of _M. m. nanus_; localities that are not represented in Fig. 1 because overlapping or crowding of the symbols would result are Italicized): WYOMING: _Big Horn Co._: Medicine Wheel Ranch, 9000 ft., 28 mi. E Lovell, 30; W slope, head of Trappers Creek, 9500 ft., 2 (USBS). _Washakie Co._: 9 mi. E, 5 mi. N Tensleep, 7400 ft., 1. _Johnson Co._: 7½ mi. W, 1 mi. S Buffalo, 6500 ft., 3; Big Horn Mountains, 3 (USBS). _Natrona Co._: Buffalo Creek, 27 mi. N, 1 mi. E Powder River, 6075 ft., 16. =Microtus montanus fusus= Hall _Microtus nanus_, Bailey, N. Amer. Fauna, 17:30, June 6, 1900 (part); Cary, N. Amer. Fauna 33:123, August 17, 1911. _Microtus montanus fusus_ Hall, Proc. Biol. Soc. Washington, 51:131-134, August 23, 1938; Warren, The Mammals of Colorado, Univ. of Okla. Press, p. 229, 1942. _Type._--Male, adult, skin and skull; No. 61281, Museum of Vertebrate Zoology; 2½ miles east of summit of Cochetopa Pass, Saguache County, Colorado; Sept. 21, 1933; collected by Annie M. Alexander; original number 2568. Type not seen by me. _Range._--Southern Colorado and northern New Mexico.