_Color._--Upper parts, in summer, near (14 _n_ to _l_) Brussels Brown or tones 1 to 3 of Raw Umber of Oberthür and Dauthenay, pl. 301, darker on top of head from nose to, or slightly behind, line connecting posterior margins of ears. Chin and usually all of upper lips white. Remainder of underparts Buff-Yellow to Straw Yellow. In winter all white except tip of tail or upper parts near (_j_) Snuff Brown or lighter than Brussels Brown with a smoked effect, with underparts white. Tip of tail at all times black. Color of underparts extends distally on posterior sides of forelegs over toes onto antipalmar faces of toes and wrists, on medial sides of hind legs to ankles over antiplantar faces of toes, distomedial third of tarsus and usually over proximal fourth to three-fourths of ventral side of tail. Least width of color of underparts averaging, in 15 males, 53 (36-69) per cent of greatest width of color of upper parts. Black tip of tail averaging 47 (38-67) mm. long. Thus averaging longer than hind foot, and 36 per cent of length of tail-vertebrae. _Skull and teeth._--Male (based on 6 adults from northeastern Oregon): See measurements and plates 19-21. As described in _Mustela frenata nevadensis_ except that: Weight, 2.9 (2.5-3.4) grams; basilar length, 40.5 (39.3-41.8). Female (based on no. 212423, adult from Vale): In so far as parts of the broken skull permit a person to judge, the skull is as described in _M. f. nevadensis_ except that: Smaller; lighter; postorbital breadth more than width of basioccipital measured from medial margin of one foramen lacerum posterior to its opposite. As compared with the skull of _M. f. nevadensis_ that of _effera_ seems, on the average, to have the preorbital part relatively smaller. Otherwise, the skull is a miniature of the skull of _nevadensis_, averaging about eight per cent smaller in linear measurements and weighs twenty-two per cent less. Comparisons of the skull with those of _M. f. washingtoni_ and _M. f. oregonensis_ are made in accounts of those subspecies. _Remarks._--This geographic race has long borne the name of _Mustela arizonensis_ (Mearns). Small size differentiates _effera_ from _nevadensis_ and specimens have been allocated to one or the other subspecies on the basis of size, or average size when several individuals are available from one locality. Complete intergradation with each adjoining subspecies is indicated by numerous specimens, more of which are assigned to these adjoining subspecies than to _effera_ itself. The minimum of size in _M. f. effera_ is found in the Blue Mountain region of northeastern Oregon. Specimens from the area intervening between these mountains and the Cascades average larger but are nearer the mean of typical _effera_ than they are to the means of _washingtoni_, _oregonensis_ or _nevadensis_.