_Remarks._--_M. e. richardsonii_ has the most extensive geographic range of any American race of _erminea_, is centrally located with respect to the other races, is more abundantly represented by study specimens in zoölogical collections than any other race, and is a sort of average for the species as a whole in most structural features. Therefore _richardsonii_ is used as a standard of comparison and accordingly is more fully described than any one of the other races each of which by reference to _richardsonii_ is described in comparative fashion. This comparative description has the virtue of more clearly indicating differences between subspecies and also makes for brevity. John Richardson, Bernard R. Ross, and names of their companions, as written on the labels of the older specimens recall to the student's mind early explorations of the north country. Edward A. Preble obtained important specimens at several places and in recent years J. Kenneth Doutt and G. G. Goodwin have made the reviser's work easier by preparing specimens in series from areas not previously well represented. The nomenclatural history of this subspecies begins with references in the literature that identify the animal as the Old World species, _Mustela erminea_--an identification which the study here reported upon shows to have been correct in the specific, although not in the subspecific, sense. Richardson, for example, in his "Fauna Boreali-Americana" published in 1829 so identified the animal. In 1838, Bonaparte, basing his description on Richardson's account of 1829, proposed the new name _richardsonii_. Richardson himself, the following year in the "Zoology of Beechey's Voyage," accepted Bonaparte's name and it has been applied to the animal in the central part of the northern timber-belt of North America ever since, except as authors used the name _Mustela erminea_ in the belief that _richardsonii_ was not distinct from _erminea_.