The tail, excepting the black tip, lags in the molt in many instances, with the result that, especially in spring, it may retain a few white hairs as late as does the belly. In autumn it is less tardy and so far as I have observed, becomes white at about the same time that the general area of the back changes color. On the tail, the black tip itself, as clearly shown in more than a score of specimens, is molted at approximately the same time in autumn as is the pelage of the body. However, the long black hairs, which appear in, say, November, appear to increase in length until January. In spring, the long black hairs of the tip of the tail seem not to be shed at the same time as the rest of the winter pelage, but remain approximately six weeks longer and then are replaced by long black hairs of the summer coat. At any rate, this is the picture presented by a half dozen specimens of _M. f. nevadensis_ and _M. f. longicauda_ which do show a spring molt to be in progress on the black tip of the tail. Schwalbe similarly (1893:536-537) has suggested that the black tip of the tail in _Mustela erminea_ in spring is not molted until about two months after the pelage on the rest of the body is changed. Schwalbe (_loc. cit._) thinks also that in _M. erminea_ studied by him, the black tip of the tail in autumn is replaced approximately one month in advance of the pelage on the rest of the body. As indicated above, my specimens of _Mustela frenata_, subspecies _longicauda_ and _nevadensis_, do not show this discrepancy in autumn. I have considered the possibility that the black tip of the tail, in some species of _Mustela_, is molted only once while the remainder of the coat was undergoing two molts. My inconclusive data lend but little support to this possibility.
The difference in pattern of color between specimens taken in autumn and spring is known to some fur-trappers of my acquaintance who have suggested that molt occurs in spring, whereas the individual hairs change color in autumn. Reference to plate 39 will show how gross comparisons might lead one to this erroneous explanation of the color change.
As to time of molt: In eight subspecies of _Mustela frenata_, namely, _noveboracensis_, _occisor_, _primulina_, _spadix_, _longicauda_, _arizonensis_, _nevadensis_ and _effera_, material is available to indicate that the autumnal molt begins in October and is completed in November, and that the spring molt occurs in March or April. A condensed list of specimens providing basis for this statement is as follows: