_Color._--As described in _Mustela erminea richardsonii_ except that underparts in summer Marguerite Yellow or more whitish; least width of color of underparts averaging, in four females from Idaho and Montana, 38 (33-43) per cent of greatest width of color of upper parts. Black tip of tail in same specimens 38 (31-42) mm. which is 57 (52-65) per cent of length of tail-vertebrae. _Skull._--Male (5 adults from Idaho County): See measurements and plates 2-4. As described in _Mustela erminea richardsonii_ except that: Weight, 1.5 (1.4-1.7) grams; basilar length, 37.0 (35.8-39.8). Female (illustrated by adult and 4 subadults in table of cranial measurements, which see): See plates 9-11. As described in _Mustela erminea richardsonii_ except that: Weight, 0.72 (0.7-0.9) grams; basilar length, 32.2 (31.6-32.8). From _fallenda_, _invicta_ differs in that the skull of the male has a relatively narrower rostrum and relatively shallower braincase. Females show the same differences but the degree of difference is about as great again as in males. The teeth are almost exactly the same size in the two subspecies. The weight is the same in males but in females _invicta_ is 18 per cent heavier. From _streatori_, _invicta_ differs in that males average larger in every measurement taken except that the anteroposterior diameter of the inner moiety of M1 is less; 36 per cent heavier; linear measurements of the skull are about 5 per cent larger and those of the teeth, with the one exception noted, about 6 per cent larger; relative to the basilar length the tympanic bullae are longer and the rostrum is relatively narrower. In females, measurements of the skull average 8 per cent more and those of the teeth 7 per cent more except that, as in males, the inner lobe of M1 is actually shorter. Females of _invicta_ are 12 per cent heavier; relative to the basilar length the skull is narrower throughout and the tooth-rows are shorter than in _streatori_. From _gulosa_, _invicta_ differs in that males average larger (about 12 per cent) in every measurement taken, excepting the anteroposterior diameter of M1 which is the same; 50 per cent heavier; relative to the basilar length the length of the tooth-rows and interorbital breadth are less. In females the inner lobe of M1 is smaller but every other measurement taken of the skull and teeth is more, _invicta_ averaging about 8 per cent larger and 22 per cent heavier; relative to the basilar length, the tooth-rows are shorter and the skull is narrower interorbitally, through the rostrum and across the zygomata.