_Type._--Male, adult, skull and skin; no. 33637, Amer. Mus. Nat. Hist.; Ironside, 4000 ft., Malheur County, Oregon; September 8, 1912; obtained by H. E. Anthony; original no. 267. The skull (plates 19-21) is complete and unbroken. The teeth all are present and entire. The skin, in summer pelage, is well made. _Range._--Upper Sonoran to Arctic Alpine life-zones of northern two-thirds of Oregon east of the Cascades, and southeastern Washington, south of the Snake River. See figure 29 on page 221. _Characters for ready recognition._--Differs from _M. f. nevadensis_ in small size, males averaging 12-1/2 per cent smaller in external measurements, 8 per cent smaller in linear measurements of skull, and 22 per cent in weight of skull, total length averaging 360 rather than 400, condylobasal length averaging 40.5 rather than 43.6; from _M. f. oregonensis_ in absence of frontonasal white patch, presence of light color of underparts on ventral face of tail and smaller skull with basilar length averaging less than 41.7 in males; from _M. f. washingtoni_ in presence of light color of underparts on ventral face of tail, in male skull by linear measurements averaging 7 (5-12) per cent shorter and relative to basilar length shorter in preorbital region and broader across mastoid processes and zygomatic arches. _Description._--_Size._--Male: Eight (6 adult and 2 subadult) males from northeastern Oregon yield average and extreme measurements as follows: Total length, 360 (340-378); length of tail, 129 (122-136); length of hind foot, 42 (40-44). Tail averages 56 (52-59) per cent as long as head and body. Length of hind foot more or less than (about same as) basal length. Female: No. 212423 from Vale, and no. 566 V. B. Scheffer, from 15 mi. E Ukiah, measure, respectively: Total length, 312, 306; length of tail, 113, 114; length of hind foot, 35, 35. Tail averages 57 per cent as long as head and body. Differences in external measurements between the one adult female and the average of the males are: Total length, 51; length of tail, 16; length of hind foot, 7. _Externals._--Longest facial vibrissae black, brown or white (often all three colors in same specimen) and extending beyond ear; carpal vibrissae same color as underparts and extending to apical pad of fifth digit; hairiness of foot-soles (in summer pelage) about as shown in stage 4 of figure 19.