E' Abdomen dark brown, like back; face uniformly brown without blackish mask; m1 with incipient metaconid; distance between upper canines less than width of basioccipital as measured between foramina situated midway along medial sides of tympanic bullae, _Mustela vison_, mink, American mink. DIAGNOSIS OF THE GENUS Genus =Mustela= Linnaeus Weasels, Ferrets, Polecats, Minks _Genotype._--_Mustela erminea_ Linnaeus. _Diagnosis._--Legs short; body relatively long; adults 190 mm. to 700 mm. in total length; skull ranging in basilar length from 16 to 70 mm.; facial angle slight; tympanic bullae greatly inflated (moderately in _Lutreola_), cancellous, and with paroccipital processes closely appressed to bullae; palate behind upper molars; dental formula: I 3 C 1 P 2-3 M 1 -, -; -, -; -, ---; -, -; inner moiety of M1 larger than outer; P4 i 3 c 1 p 3-2 m 2 with simple deuterocone; in m1 inner moiety of M1 larger than outer; P4 with simple deuterocone; in m1 trigonid longer than talonid, metaconid absent (incipiently developed in _Lutreola_), and talonid trenchant. For many years prior to 1911, the name _Mustela_ was applied to martens, and _Putorius_ was regarded as the first available generic name for the weasels. In 1911 Thomas (1911:139) showed that _M. erminea_ (_Mustela_ of Gesner) by tautonymy was the type of _Mustela_ and subsequently the generic name _Mustela_ has been used for the true weasels which include the American weasels to which we now apply the specific names _erminea_, _rixosa_ and _frenata_. The mink, _Mustela (Lutreola) vison_, and the black-footed ferret, _Mustela (Putorius) nigripes_, since 1911 also have been referred by most American authors to the genus _Mustela_, the names _Lutreola_ and _Putorius_ being regarded by these authors as having no more than subgeneric status. European writers, on the other hand, accord greater taxonomic weight to the zoölogical differences between ferrets and weasels and, therefore, accord full generic rank to _Putorius_. Consequently, for the black-footed ferret, Europeans today write _Putorius nigripes_ and Americans write _Mustela nigripes_. For the same reasons, the name of the mink is written by some European zoölogists _Lutreola vison_ and by American zoölogists _Mustela vison_. EXPLANATION OF SYSTEMATIC TREATMENT For each full species there will be found under the account of it the following information: Type, statement of geographic range, selected characters for ready recognition, other characters of the species, a summary of geographic variation, and information on habits, in the order mentioned.