The name _richardsonii_ was based by Bonaparte on Richardson's description which in turn was drawn from a specimen taken at Fort Franklin, that thus becomes the type locality. It is fortunate that Preble, in 1903, succeeded in taking specimens there because the place is near the belt of intergradation between _arctica_ and _richardsonii_. Of Preble's two adult males (see Preble, 1908:232) I have examined no. 133847, which is in transitional pelage and therefore gives no clue in so far as coloration is concerned, as to affinities with _arctica_ versus _richardsonii_. Specimens in the summer pelage are much to be desired from Fort Franklin. Regardless of what their coloration may be, specimen no. 133847, in external measurements and most certainly in cranial features is of the race to the south and not the race that Merriam named _arctica_. Because all specimens from localities to the south of Fort Franklin likewise differ from _arctica_ of the barren grounds, considerable additional confidence is felt in allocating the name _richardsonii_ to the animal which ranges from Fort Franklin southward rather than to the one, here designated _arctica_, that occurs to the northward of Fort Franklin. Although in most structural features _richardsonii_ is a sort of average for the American races of the species, it is the extreme in high degree of sexual dimorphism. The difference in size between the males and females is greater than in any other race except possibly _M. e. kadiacensis_ in which so little is known of the female that the difference between the two sexes cannot be accurately judged. It will aid in understanding the high degree of secondary sexual difference in _richardsonii_ to visualize two kinds of weasels distributed over the northern half of the continent, thinking now of the geographic area in America occupied by the whole species _Mustela erminea_ of which the subspecies _richardsonii_ is only a part. One of the two kinds of weasel is the male ermine and the other the female. The decrease in size of the male, as measured by the weight of the skull, is in the ratio of 7 in the north to 2 in the south. This decrease is gradual whereas the corresponding decrease from 3 to 1 in the female is not gradual; half of the decrease in the female occurs in the short north to south distance comprised in the belt of intergradation, along the northern boundary of _richardsonii_, between it and _arctica_. As a result _richardsonii_ is composed of females with medium sized skulls and males with relatively large skulls, the ratio by weight being approximately 5 to 2. The disproportion in races of ermines both to the north and to the south is less. Actually in the north (_arctica_) the approximate ratio by weight is 2-1/3:1; in _richardsonii_, 2-1/2:1; in the south (_muricus_), 1-2/5:1. Indicated in still another way in _richardsonii_ the skull of the female is 56 per cent lighter than that