_Color._--Winter pelage all white except tip of tail. Summer pelage with upper parts uniform in color and Raw Umber or darker (16_n_) of Ridgway and about tones 2 to 3 of Chocolate of Oberthür and Dauthenay, pl. 343, but in autumn some specimens have more light red than tones 2 or 3. Underparts Sulphur Yellow, Colonial Buff, or Primrose Yellow, often white on chin and insides of forelegs; color of underparts extends narrowly over upper lips, distally on posterior sides of forelegs onto antipalmar surface of forefeet, onto proximal two-thirds or three-fourths of underside of tail as length of tail is measured along tail-vertebrae, on medial sides of hind legs to a point between knee and ankle but reappears on antiplantar faces of toes and in some individuals is narrowly continuous onto toes; rim of ear in some specimens with short, white or pale hairs giving ears distinct whitish border; least width of color of underparts averaging, in adult males from Alaska, 65 (46-93) per cent of greatest width of color of upper parts. Black tip of tail in 5 males in winter pelage from Tanana averaging 84 (70-93) mm. which is 91 (75-107) per cent of length of tail-vertebrae. _Skull._--Male (based on 5 adult topotypes): See measurements and plates 2-4. As described in _Mustela erminea richardsonii_ except that: Weight, 3.5 (3.1-3.9) grams; basilar length 42.5 (41.8-43.3); length of tooth-rows more than length of tympanic bulla; breadth of rostrum measured across lacrimal processes averaging more than a third of basilar length; interorbital breadth more than distance between glenoid fossa and posterior border of external auditory meatus; zygomatic breadth more than distance between last upper molar and jugular foramen. Female (based on 2 adult topotypes and 2 adults and 4 subadults from central Alaska): See measurements and plates 9-11. As described in _Mustela erminea richardsonii_ except that: Weight, 1.5 (1.2-2.0) grams; basilar length, 35.7 (34.5-37.0); length of tooth-rows more than length of tympanic bulla; breadth of rostrum more than 30 per cent of basilar length; interorbital breadth more than distance between glenoid fossa and posterior border of external auditory meatus; zygomatic breadth more than distance between last upper molar and jugular foramen (except in specimens from Ellesmere Island where two distances are approximately equal). Cranial differences from _Mustela erminea kaneii_ (which occurs on the Asiatic side of Bering Strait), in both males and females, are: larger size relatively as well as actually, broader except in mastoidal region where relatively (to basilar length) the width is less; preorbital part of skull broader as well as longer.