Some specializations are obvious in _Mustela africana_. One is the reduction in number of premolars; p2 is absent whereas it is normally present in the other weasels; P2 has one instead of two roots; and, in relation to the other teeth, m2 is smaller. The shortness of the preorbital part of the skull in relation to the length of the skull as a whole may reflect the mentioned reduction of the premolars or retention of a primitive shape of skull, or both. Also, certain features which denote immaturity in other weasels are retained in adults of this species, as for example, sutures on the dorsal face of the preorbital region of the skull. [Illustration: FIGS. 17-22. Views of the feet of American weasels (subgenus _Mustela_) to show differences in number and arrangement of the pads and variation in degree of hairiness of the soles. × 1-1/2 In each figure, left-forefoot on left, and left hind foot on right. FIG. 17. _M. rixosa rixosa_, Halifax, N.S.; juv., [F], 7425 U.S.N.M. Fig. 18. _M. erminea richardsonii_, Ft. Chimo; ad. [F], 14866 U.S.N.M. Fig. 19. _M. frenata noveboracensis_, Mich., July 7, 1913; ad. [M], 44689 M.Z. Fig. 20. _M. f. frenata_, Brownsville, June 1, 1892; yg. [M], 34043 U.S.N.M. Fig. 21. _M. frenata panamensis_, Panamá, February 17, 1911; sad. [F], type. Fig. 22. _M. a. africana_, Pará, Brazil, Sept., 1908; yg. [M], 37475 A.M.N.H. Figs. 17, 18 and 19. Drawn from specimens preserved in alcohol. Figs. 20, 21 and 22. Drawn from relaxed feet of dried skins.] _Mustela africana_, all characters considered, is the most aberrant of the American weasels. That is to say, greater difference prevails between _M. africana_ and any other American weasel than exists between any other two American weasels. The distinctive cranial and dental characters, excepting the reduction in number of premolars, are of a primitive nature. For example, the relatively wide postorbital region, the large braincase that is inflated anteriorly, and the flattened tympanic bullae are points of resemblance to the holarctic _Mustela erminea_, the species which is regarded as most closely resembling the stem form. Also, the mentioned characters in adults of _M. africana_ resemble ontogenic stages passed through by other weasels. Consequently, it is thought that _M. africana_ crossed the filter-barrier from North America to South America, remained isolated from the original stock for a length of time sufficient to permit _africana_ to differentiate from North American weasels and _vice versa_ to such a degree that crossbreeding with the _frenata_ stock was prevented when _frenata_, at a later time, pushed southward over the, then zoölogically less-effective, water barrier, or continental bridge if it was by this time in existence. [Illustration: FIG. 23. Diagram indicating probable relationships of the species of American weasels.]