_Description.--Size._--Male: Seven adult topotypes yield average and extreme measurements as follows: Total length, 278 (249-305); length of tail, 77 (69-81); length of hind foot, 36.5 (34-40). A male topotype of unknown age weighed 113 grams.
Female: Two adult topotypes, with actual measurements in parentheses, average as follows: Total length, 232 (228-236); length of tail, 60 (57-62); length of hind foot, 27 (27-27). An adult from Morovitz Guard Station, Wash., weighed 54 grams.
_Color._--Winter pelage rarely white, brown pelage indistinguishable from summer pelage except for slightly more smoky tinge in winter in specimens from some localities; otherwise as described in _Mustela erminea streatori_ except that least width of color of underparts averaging, in seven adult topotypes, 18 (0-37) per cent of greatest width of color of upper parts. Black tip of tail averaging, in same series, 45 (38-52) mm. which is 58 (53-65) per cent of length of tail-vertebrae.
In comparison with _richardsonii_ and _invicta_, _fallenda_ differs in darker color of upper parts and their extension at the expense of the light-colored underparts which are narrower by a half. In correlation with this restriction in area of the light-colored underparts, the upper lips are brown instead of white. In comparison with _anguinae_, _olympica_ and _streatori_, the longer black tip on the tail is the principal difference in color. From _gulosa_, _fallenda_ differs in slightly darker color of upper parts and in narrow underparts, the width of the same being only about a fifth instead of a third of the width of the dark-colored upper parts.
_Skull._--Male (based on 7 adults): See measurements and plates 5-7. As described in _Mustela erminea richardsonii_ except that: Weight, 1.5 (1.3-1.7) grams; basilar length, 35.7 (34.3-38.2).
Female (based on 6 ads.): See measurements and plates 11-13. As described in _Mustela erminea richardsonii_ except that: Weight, 0.85 (0.73-1.0) grams; basilar length, 30.6 (29.4-31.7); breadth of rostrum more than 30 per cent of basilar length.
In comparison with _richardsonii_, skulls of males differ as follows: averaging smaller in every measurement taken with no overlap in several dimensions; 40 per cent lighter; in relation to basilar length, rostrum (orbitonasal length) longer and skull slightly broader interorbitally. Females average smaller in every cranial and dental measurement taken with no overlap in basilar length, length of tooth-rows and length of tympanic bulla; 22 per cent lighter; breadth of rostrum more, rather than less, than 30 per cent of basilar length; in relation to basilar length, pre-and interorbital parts of skull larger, and mastoid breadth more.