Some other early names thought by Zimmermann (1943:290) to have been based on the dwarf weasel of Europe are judged to be nomina nuda and therefore are to be ignored.
The name Mustela minor Nilsson 1820 was thought by Miller (1912:402) to be a renaming, and hence a synonym, of Mustela nivalis Linnaeus. If that is the case the name does not apply to the dwarf weasel. If the name Mustela minor Nilsson was instead based on the dwarf weasel, the name might still be unavailable, depending on rulings on secondary homonyms, because the name might be preoccupied by [Lutra] minor Erxleben 1777 which is a synonym of [Mustela] lutreola Linnaeus 1766. Two names seemingly available for weasels, and in use for them today, which might replace rixosa as the name of the species, are, first, Mustela boccamela Bechstein, 1801, of Sardinia [= Mustela nivalis boccamela of Miller, 1912, 405] and second, Putorius numidicus Pucheran, 1855, of Morocco and Algeria [= Mustela numidica of Allen, G. M., 1939, 183]. As they stand in the current literature, Mustela numidica is a species distinct from the dwarf weasel and the other name, Mustela nivalis boccamela, is an insular subspecies of the mouse weasel. Zimmermann (1943:292), however, implies that M. numidica may belong to the dwarf weasel group when he says "Ob auch iberica Barr.-Ham. als Unterart zu minuta Pom. zu stellen ist, soll hier nicht untersucht werden, ebensowenig die von Cabrera vermutete Zugehörigkeit der grossen nordafrikanischen M. numidica Puch. zur 'iberica-Gruppe'." The answer to this problem requires a taxonomic, rather than a nomenclatural, decision. Whether either M. numidica or M. boccamela are conspecific with the dwarf weasel I cannot at this time ascertain for want of adequate specimens. Because these two names, M. boccamela, and M. numidica, are assigned to kinds of weasels which are currently regarded as specifically distinct from the dwarf weasel, and because all the other names which certainly have been assigned to Old World populations of the dwarf weasel before 1896, so far as I know, are nomina nuda or are preoccupied, the next available name, Mustela rixosa (Bangs, 1896), is here employed.
Remarks.—This species may have a wider geographic range in northeastern North America than is now known. Strong (1930:7) writes that the Naskapi Indians of the interior country of Labrador between Hamilton Inlet and Ungava Bay "have only one name for weasel, mé-tah-kwut, but they say there are three kinds in their territory, a large, an intermediate, and a very small weasel. The latter suggests the least weasel . . . which has not been recorded from northern Labrador."
In the northern part of the range of the species, the winter pelage is white and the summer pelage is brown. In the southern part of the range, that is in the range of the subspecies allegheniensis, the winter pelage is either brown or white and the time of the molt into winter pelage is irregular; each of eleven individuals from Pennsylvania, Michigan and Ohio, taken in December, January, February and March is mostly white but retains some considerable part of the brown pelage of the previous coat on top of the head and usually also along the midline of the entire dorsum. These eleven animals include individuals of each sex. Of each sex, some are adults and some are subadults. Therefore, the delayed or incomplete fall molt, at present, cannot be correlated with either sex or with any particular age. No wild-taken specimens of M. erminea or of M. frenata of the same region show this delayed or incomplete molt.
Possibly this delay or incompleteness of molt is the result of the same cause that lies behind the birth of some M. rixosa in midwinter. As listed below, several litters of young have been found in midwinter. In fact it appears that in the United States, young may be born in every month of the year although, according to existing information, more litters are produced in spring and in winter than in summer and autumn. Many juveniles and young of allegheniensis examined in study collections clearly were born in spring but about as many seem to have been born in midwinter as at any other time (in the light of present knowledge) and this is in contrast to what we know of the two other species of American weasels since their young, so far as known, are born in spring.
One instance is worthy of detailed comment. An adult female, no. 783 Ohio State Museum, taken on January 31, 1931, at Vinton, Meigs County, Ohio, bears the following notation on the attached label "nest plowed out of ground. Very small young escaped—marked like parent. ♀ was nursing." The enlarged mammae on the dried skin substantiate the statement that the female was nursing young. She has a brown mask continuous from one ear through the eye, across the forehead and through the other eye to the opposite ear. On each side of the body a stripe of brown 5 to 10 mm. wide extends from the upper part of the foreleg back to the thigh and base of the tail, uniting there with its opposite and covering the tail. There are a few spots of brown on the shoulders, and rump and one on the middle of the back. Otherwise the specimen is white. One implication of the statement on the label that the young which escaped were marked like the parent (presumably this female parent) is that this female is a partial albino. I am more inclined, however, to the view that there was an unseasonable activity of the particular glands of internal secretion the hormones of which promote embryonic growth and that these glands, or others controlled by them, were in some way responsible for an abnormal progress of molt, or for a reversal of molt in that one molt began before the previous molt had been completed.
Excepting this one specimen, no. 783 from Vinton, Ohio, all of those in transitional pelage indicate that the direction of the molt pattern is the same as in M. frenata and M. erminea. That is to say, the autumnal molt begins on the midventral line and the molt in spring begins on the mid-dorsal line. Furthermore, the normal progress of each molt appears to follow the same pattern that has been described above for Mustela frenata.