_Characters for ready recognition._--Differs from _M. e. anguinae_, in males, by lesser average size, hind foot ordinarily less than 33.4, and interorbital breadth ordinarily less than 8.5, in females by smaller size, total length less than 235, tail less than 65, hind foot less than 27.5, basilar length less than 30.2; from _M. e. fallenda_, in males, by length of hind foot less than 33, black tip of tail less than 36.5, weight of skull less than 1.2 grams, basilar length less than 33.5, in females length of hind foot less than 25.5, weight of skull less than 0.6 grams, basilar length less than 28.4; from _M. e. streatori_ by smaller size, in males hind foot less than 33.0, basilar length ordinarily less than 32.5, in females by hind foot ordinarily not longer than 24, by breadth of rostrum less than 8.6, depth of braincase at posterior border of upper molars less than 7.6. _Description._--_Size._--Male: Twelve individuals of adult proportions yield average and extreme measurements as follows: Total length, 243 (205-269); length of tail, 65 (60-74); length of hind foot, 31 (29-32). Female: Corresponding measurements of six females are: 196 (188-208), 52 (45-60?), 23.4 (22.7-24.0). An adult weighs 30 grams. _Color._--As described in _Mustela erminea streatori_ except that least width of color of underparts averaging, in 12 males of adult proportions, 5 (0-11) per cent of greatest width of color of upper parts. Black tip of tail averaging, in same series, 26 (20-35) mm., which is 40 (31-58) per cent (average the same as in _streatori_) of length of tail-vertebrae. _Skull._--Male (based on 5 adults): See measurements and plates 5-7. As described in _Mustela erminea richardsonii_ except that: Weight, 1.0 (0.9-1.1) grams; basilar length, 31.8 (30.6-32.5); length of tooth-rows more or less than (about equal to) length of tympanic bulla. Female (illustrated by 3 adults): See measurements and plates 12-14. As described in _Mustela erminea richardsonii_ except that: Weight, 0.55 (0.52-0.58) grams; basilar length, 27.1 (26.7-27.5); breadth of rostrum more than 30 per cent of basilar length. In comparison with _streatori_, skulls of corresponding sex average smaller in every measurement taken with no overlap in most of those of females. Exception is to be made for the inner lobe of M1 in males where the size is the same. By weight males are smaller by 10 per cent and females by 14 per cent. In relation to other parts of the skull the tympanic bullae are narrower and in females they are shorter as well. Comparison with _anguinae_ and _fallenda_ has been made in the accounts of those subspecies.