Fig. 17. M. rixosa rixosa, Halifax, N.S.; juv., ♀, 7425 U.S.N.M.
Fig. 18. M. erminea richardsonii, Ft. Chimo; ad. ♀, 14866 U.S.N.M.
Fig. 19. M. frenata noveboracensis, Mich., July 7, 1913; ad. ♂, 44689 M.Z.
Fig. 20. M. f. frenata, Brownsville, June 1, 1892; yg. ♂, 34043 U.S.N.M.
Fig. 21. M. frenata panamensis, Panamá, February 17, 1911; sad. ♀, type.
Fig. 22. M. a. africana, Pará, Brazil, Sept., 1908; yg. ♂, 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.