Weasels of the subgenus _Mustela_ are known from the Pleistocene but not from deposits laid down at an earlier time (see page 10). The Pleistocene weasels from Rancho La Brea of southern California and from Potter Creek Cave and Samwel Cave, both of northern California, are subspecifically indistinguishable from the weasels living in those same localities today. The other notable occurrence of weasels in the Pleistocene is in the Conard Fissure of Arkansas. Brown (1908:181, 182, pl. 17) names two kinds from the Fissure. One is an extinct subspecies (_Mustela frenata gracilis_) possibly of the species which occurs in the same region today and the other, _Mustela erminea? angustidens_, is an extinct subspecies of a species which occurs only farther north today. _M. erminea_ came south, probably in front of one of the ice sheets, as did several other species of American mammals, now of more northern distribution, that left their remains in Conard Fissure. _Mustela rixosa_ is not recorded as a fossil in America although it is known from the "Diluvial" deposits of the Old World; see Woldrich (1884:1000), who employs the name "_Foetorius minutus_ n. sp.," and see also Zimmerman (1943:295-296). The ermine, _Mustela erminea_, is the most generalized of the full species. For example, the number of teeth is as large as in any other species and greater than in certain species. The teeth are sharp-pointed, uncrowded, and individually less specialized than in any other American weasel. M1 has the inner half, or lobe, of approximately the same size as the outer lobe instead of much larger than the outer lobe (the outer lobe is the larger in several other species). The tympanic bullae are less inflated and less protruded from the braincase. The skull is rounded, and has no marked crests and ridges whereas the skulls of the other species are more pronouncedly modeled and sculptured. Therefore, it is possible to think of these other species as derived from _M. erminea_. A derivation in the reverse direction would be more difficult. From the foot soles of an ermine, or a weasel closely resembling an ermine, the more complex soles of _Mustela africana_ could have been derived by a decrease in hairiness, although it would be necessary to suppose that the thenar pad has been retained in _africana_ and has been lost in the living _erminea_. The alternate possibility, namely, that the thenar pad was a relatively recent acquisition in the _africana_ line seems less probable. The tail of _erminea_ is of "average" length and in size of entire animal _erminea_ is intermediate between the other American weasels. Structurally, _Mustela erminea_ appears to be nearest the stem form from which all of the living weasels ascended. Its present holarctic distribution is in harmony with the view that it is a direct descendant from the stem form because the stem forms of most of the known kinds of mustelids appear to have lived in the holarctic region. To be sure,