_Characters for ready recognition._--Differs from _Mustela rixosa_ in presence of black pencil on tail, tail-vertebrae more than a fourth of length of head and body, and in regions where the two species occur together, basilar length of skull more than 32.5 in males and more than 31.0 in females; from _Mustela frenata_, in regions where the two species occur together, by tail less than 44 per cent of length of head and body and by postglenoidal length of skull more than 46 per cent of condylobasal length in males and more than 48 per cent in females.
_Characters of the species._--Size medium to small (total length 225 to 340 mm. in males and 190 to 290 mm. in females); tail 30 to 45 per cent of length of head and body, with distinct black pencil; caudal vertebrae 16 to 19; skull with long braincase and short precranial portion; postglenoidal length, when expressed as a percentage of the condylobasal length, more than 48 in females and ordinarily more than 46 in males; upper parts brown; underparts whitish, ordinarily continuous from chin to inguinal region but in subspecies in the humid region along the Pacific Coast interrupted in some individuals by brown of upper parts encircling body in the abdominal region. The soles of the feet in each of the subspecies are densely haired in winter and have only a relatively small area of the foot-pads exposed in summer, the intervening areas being well haired even at that season. The uniformity throughout the species as regards hairiness of the foot-soles and also the character of the vibrissae makes it unnecessary to describe these features in the accounts of the subspecies of _erminea_.
_Geographic variation._--In the Old World 16 or more subspecies are currently recognized and there are 20 in North America. The features in which geographic variation is especially prominent are: First, size, as expressed by external measurements and weight, second, color pattern, depending on the extent, in relation to one another, of the dark-colored upper parts and light-colored underparts, and third, breadth and depth of the rostral region of the skull. Except in size, the variation in the skull is less than in _M. frenata_. Likewise in tone and shade of upper parts and hue or tint of underparts, _erminea_ is less variable than _frenata_ and has the face all of one color without the contrasting color-pattern of the face and head seen in many subspecies of _frenata_. _M. erminea_ exceeds _frenata_ as regards variation of the size of the area occupied by the light-colored underparts. At one extreme is the subspecies _arctica_ in which the area of the light color extends well up on the sides of the body, down the insides of the legs, over the feet and far out on the lower side of the tail whereas at the other extreme are the races _streatori_ and _olympica_ in which the light-colored underparts are restricted to two areas, one on the chin, throat and chest, and the other on the inguinal region. These areas may or may not be connected by a thin line of white color along the midline of the underparts. In size of animal, _erminea_ probably exhibits the maximum variation among American species of weasels; an average-sized male of the race _arctica_ weighs 4 times as much as one of the race _muricus_, and in the species _frenata_ I doubt that the difference is quite as great between individuals of the smallest race, _effera_, on the one hand, and either of the largest races, _texensis_ or _macrophonius_, on the other hand although actual weights are not available for these races of _frenata_. As elsewhere indicated, the small-sized individuals of _M. erminea_ are of the southern races and the large-sized individuals are of the northern races. This decrease in size southward occurs both in Asia and in America.