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.
For each subspecies, information is presented in the following order: earliest available zoölogical name, synonyms, type, geographic range, zoölogical characters for ready recognition, description (mentioning size, certain external features including color, the skull and teeth) historical material when warranted, remarks which may elaborate on points made in preceding paragraphs, and other information thought to be useful, and finally a list of specimens examined.
In explanation of certain of these categories it should be said that in the synonymy no attempt is made to list every published reference to the subspecies concerned. It is aimed, however, to include at least one citation to each name-combination that has been applied, to the subspecies concerned, along with other especially important references. Mere records of occurrence are not regarded as especially important and citations to them ordinarily are omitted in the synonymy. No comma is placed between the zoölogical name and the name of the author who coined and first used the name in accordance with the rules of zoölogical nomenclature. Otherwise a comma is interposed between the zoölogical name and the name of the user (author). When the accepted (earliest available) name of a subspecies at the head of any one of the following accounts is combined with a generic name different from that with which it originally was placed, the authority for the name is set in parentheses. The same rule is followed with the name of a full species when it is written without any subspecific name following. Parentheses in such situations, therefore, denote that for the terminal part of the scientific name there has been a change in generic name with which the terminal part of the scientific name is here associated.
In the paragraph headed "characters for ready recognition," only a few characters, namely, those regarded as most useful for identification when the student has limited time, are mentioned. Other features useful for distinguishing the kind of animal in question from its near relatives are to be found in the description and comparisons.