The secondary sexual variation in shape of the skull consists of a slenderness in the female. In relation to the basilar length the spread of the zygomatic arches is more in males and, except in the one subspecies _M. f. altifrontalis_, the rostrum is broader. Also the interorbital region is relatively broader in males of most subspecies. In most subspecies of both _M. frenata_ and _M. erminea_ the tympanic bullae are relatively (to the basilar length) longer in females. The maximum sexual dimorphism occurs in _M. erminea arctica_ and the minimum dimorphism in _M. e. haidarum_, _M. e. anguinae_ and _M. e. muricus_. Taking into account all of the subspecies of each of the North American species, the shape of the skull differs most in _M. erminea_ and least in _M. frenata_. In the latter species the greatest difference in shape of the skull, as was true also of its weight, is in the subspecies _M. f. noveboracensis_. In these two subspecies, _M. f. noveboracensis_ and _M. e. arctica_, in addition to the secondary sexual variation already mentioned in the skull, females have the braincase smoother and more rounded, the postorbital-, mastoid-, and lacrimal-processes relatively smaller, and the ventral face of the tympanic bulla at its anterior margin more nearly flush with the floor of the braincase. In the weasels, subgenus _Mustela_, the disparity in size of the two sexes is almost or quite as much as in any other fissiped carnivore. It is because of this large degree of difference that the skulls of the two sexes are described separately in the following systematic accounts. The need for such treatment was recognized by Reinhold Hensel (1881:127) more than sixty years ago when he wrote in the introduction to his "Craniologische Studien," of _Mustela_, as follows: ". . . die Geschlechtsdifferenzen am Schädel vieler Säugethiere . . . so gross sind, dass man diese wie Schädel verschiedener species behandeln muss, während in anderen Ordnungen (Rosores, Edentaten) die Schädel solche Unterschiede nichtzeigen." In the past, failure to appreciate the large amount of secondary sexual variation has resulted in erroneous deductions as regards characters of certain geographic races and has been the cause of some nomenclatural confusion, as for example, in _Mustela frenata macrura_, where the female was named as a separate species (_Mustela jelskii_). Individual Variation Individual variation is here considered to be the variation in one species which can occur between offspring of a single pair of parents, after variation ascribable to differences in age, sex, and season is excluded. Individual variation, therefore, is a term here used in a composite sense; it includes variations which probably represent different genetic strains within certain populations and variations induced within one generation by environmental factors.