Today, _erminea_ is not known to occur nearer Conard Fissure than northern Iowa, more than 400 miles to the northward. In comparison with the race there, _bangsi_, males of _angustidens_ are of approximately the same size but in the shorter distance between the glenoid fossa and anterior margin of the tympanic bulla, and also in the lesser postglenoidal length of the skull, _angustidens_ resembles the northernmost American subspecies of _erminea_. Females of _angustidens_ differ more from any living weasel than the males do. The females are much larger than those of _bangsi_, and among living American races of _erminea_ most closely resemble intergrades between _arctica_ and _richardsonii_ which intergrades are found approximately 1700 miles to the north of Conard Fissure. In females, the preorbital part of the skull in _M. e. arctica_ is broader and in _M. e. richardsonii_ narrower than in _angustidens_. If it seems strange that females of _angustidens_ resemble one subspecies whereas males, in size, resemble another subspecies almost a thousand miles distant, it should be remembered that the degree of sexual dimorphism varies much from one subspecies to another in the Recent animals. An example is furnished by _Mustela erminea fallenda_ and _Mustela erminea invicta_. The assemblage of mammals from Conard Fissure includes several species of boreal predilections which, like _Mustela erminea_, now occur only much farther north than Arkansas. At one time the edge of the sheet of ice was only about 200 miles north of Arkansas. It may be significant that the cranial characters of the female ermine from the Fissure, and qualitative cranial characters of males from there, are most nearly approximated among Recent weasels by those which live along the southern edge of the frozen tundra. In view of what has been said, the possibility should be considered that the distinctive cranial features of _angustidens_ may be the result of evolutionary change in time as well as of geographic variation resulting from horizontal placement. =MUSTELA RIXOSA= (Bangs) Least Weasel (Synonymy under subspecies) _Type._--_Putorius rixosus_ Bangs, Proc. Biol. Soc. Washington, 10:21, February 25, 1896. _Range._--From Norway and Switzerland eastward through Siberia and all the way across North America, but unknown from Iceland, Greenland and the Arctic islands west of Greenland; in North America, from the Arctic Life-zone south to Central British Columbia, Montana and into parts of the Upper Austral Life-zone as in the eastern half of the continent.