The southern extension of range in the Appalachians (to North Carolina) is not duplicated in the Rocky Mountains of western North America probably because the region there suitable for _rixosa_ south of Central British Columbia and Montana is occupied by the almost equally small _Mustela erminea muricus_ and related subspecies which seem to fill the ecological role that _rixosa_ plays where it occurs. The small size of females of _M. erminea cicognanii_ in New England may similarly account for the absence of _rixosa_ there. _Characters for ready recognition._--Differs from both _Mustela erminea_ and _Mustela frenata_ by tail a fourth or less of length of head and body and without a black tip (at most a few black hairs at extreme tip in rixosa), and from _M. frenata_ and from _M. erminea_ in regions where it and _rixosa_ occur together, by basilar length of skull less than 32.5 in males and less than 31.0 in females. _Characters of the species._--Size small: Total length less than 250 in males and 225 in females; tail a fourth or less of length of head and body, and without a black pencil and at most with a few black hairs at extreme tip; caudal vertebrae 11 to 16, normally 15 in _M. r. rixosa_, and 11 in one _M. r. eskimo_ examined; skull with long braincase and short precranial portion, thus essentially same shape as in _M. erminea_ but the largest males of _M. rixosa_ always with a lesser basilar length that even the smallest females of _M. erminea_ or _M. frenata_ of the same geographic area. In fact no specimens of _M. frenata_ have skulls so small as the largest _M. rixosa_, and skulls of equal size of _M. erminea_ and _M. rixosa_, for example, _M. erminea muricus_ of Colorado and _M. rixosa eskimo_ of Alaska, differ in that when the skulls are viewed from directly above those of _rixosa_ have the mastoid processes more prominent, or the braincase is higher in relation to its width or both differences together prevail. Stated in another way, comparison of skulls of equal size of _rixosa_ and _erminea_ shows that in the latter the braincase is more nearly flat and is wider above and in front of the mastoid processes; therefore, the greatest breadth of the braincase equals or exceeds the mastoid breadth, whereas the reverse is ordinarily true of _rixosa_.