Female (based on 2 adults, one from Carlotta and one from 13 mi. SW Grants Pass): See measurements and plates 34-36. As described in _Mustela frenata nevadensis_ except that: Weight, 2.4 (2.2-2.6) grams; basilar length, 37.7 and 39.5; zygomatic breadth less than distance between condylar foramen and M1 and less than distance between anterior palatine foramen and anterior margin of tympanic bulla. See under "_Remarks_" for additional data on variation in size of skulls of females. The skulls of the female averages 31 per cent lighter than that of the average male. Because there is much geographic variation between specimens here referred to _oregonensis_, the person who is guided by the present account should keep in mind that results, here reported, of comparisons of the skull with those of other races, were obtained by employing specimens of _oregonensis_ from Carlotta and Eureka, California. These specimens from California are judged to have more of the characters of the subspecies _munda_ than do specimens of _oregonensis_ from more northern localities. Compared with that of _M. f. washingtoni_ the skull of the male is shorter, especially in the preorbital region and is relatively broader across the mastoidal processes and zygomatic arches. The skull of the female is longer in the preorbital region, has a less cylindrical braincase and differs less from the male skull than is the case in _M. f. washingtoni_. Compared with _M. f. effera_, the skull of the male is smaller in every part measured and relative to the basilar length is broader across the mastoids and has relatively shorter tympanic bullae. From _M. f. nevadensis_ the skull of the male differs in the same way except that size is about the same. The skull of the female _oregonensis_ is more heavily ridged and is relatively broader across the mastoids than that of _effera_. From _M. f. saturata_, _oregonensis_ is not surely known to differ in cranial characters. From _M. f. munda_, _oregonensis_ differs in having the skull of both sexes smaller, and on the average, in all parts measured, has a less marked postorbital constriction, relatively narrower interorbital region and relatively more expanded zygomata. From _M. f. altifrontalis_, males of _oregonensis_ differ on the average, in having larger teeth, and relative to the basilar length, a greater mastoid breadth and a shallower braincase as measured at the anterior margin of the basioccipital. Females of _oregonensis_ differ in larger average size of skull, except for breadth of rostrum and interorbital breadth which, therefore, are relatively less in _oregonensis_, as also is the relative depth of the skull measured at the posterior borders of the upper molars and at the anterior margin of the basioccipital. However, skulls of females of _oregonensis_ have relatively longer tooth-rows and are relatively broader across the zygomata and mastoidal processes.