_Externals._--As described in _Mustela frenata nigriauris_. _Color._--Spot between eyes, narrow band or spot confluent with color of underparts on each side of head anterior to each ear, chin, lower lips, and rarely posterior third or less of each upper lip white; dark spot posterior to each angle of mouth uniformly present and of large size; tip of tail black; remainder of upper parts near (14 _l_) Sudan Brown and tone 4 of Raw Umber of Oberthür and Dauthenay, pl. 301; occasionally, slightly darker brown on forehead, nose, and about eyes. Underparts near (_a_ to _c_) Ochraceous-Buff and sometimes Orange-Buff. Color of underparts extends distally on posterior sides of forelegs over toes onto antipalmar faces of feet and wrists, on medial sides of hind limbs over antiplantar faces of toes. Least width of color of underparts averaging, in a series of 5 males from Mendocino County, 57 (46-67) per cent of greatest width of color of upper parts; 38 (35-40) in 3 males from Point Reyes, Marin County. Black tip of tail in Mendocino County series averaging 53 (46-60) mm., which is same length as hind foot and 32 per cent of length of tail. In Point Reyes males, black tip of tail averages 44 (34-52) mm., which is less than length of hind foot and 45 per cent as long as tail-vertebrae. Several specimens of the smaller, inland variant (see under "_Remarks_") are near (_l_) Antique Brown rather than near (14 _l_) Sudan Brown above and hence do not differ in this respect from _nigriauris_. _Skull and teeth._--Male (based on 3 adults from Mendocino County): See measurements and plates 19-23, 30. As described in _Mustela frenata nigriauris_ except that: Weight, 6.0 (5.4-6.3) grams; basilar length, 47.6 (46.5-48.2); length of tympanic bulla more than length of lower molar and premolar tooth-row. Female (based on no. 19723, M.V.Z., from Point Arena): See measurements and plates 34-36, 40. As described in _M. f. nigriauris_ except that: Weight, 3.0 grams; basilar length, 42.3. The skull of the female is 50 per cent lighter than that of the average male.