Variations in the tympanic bullae provide many characters useful in distinguishing weasels from different localities. Most of these characters have to do with degree of inflation of the bullae. Indirectly correlated with degree of inflation is first the extent of removal of the anterior margin of the bulla from the glenoid fossa and foramen ovale, and second the form (convex, flat, or concave) of the part of the squamosal bone between the foramen ovale and the anterior margin of the tympanic bulla. As one proceeds southward from, say, southwestern Kansas through the geographic range of the species _Mustela frenata_, there is a progressive deflation of the bulla, an increase in length of the space between its anterior margin and the foramen ovale, and the floor of the braincase in front of the bulla changes from ventrally concave to ventrally convex. (See figs. _e_ and _h_ of pl. 24 and figs, _e_ and _f_ of pl. 27.) One extreme of this variation in bulla is shown in _Mustela frenata neomexicana_ (fig. _e_ of pl. 24), in which the anterior margin of the bulla (viewed from the ventral side) rises vertically from the floor of the braincase to form a 90-degree angle. The other extreme, the uninflated bulla, is in _Mustela frenata panamensis_ (fig. _e_ of pl. 27), in which the anterior margin of the bulla is not raised above the floor of the braincase. This variation is remarkable because it occurs within a single species. Otherwise, in the family Mustelidae, differences in the tympanic bullae as great as that between the two subspecies _M. f. neomexicana_ and _M. f. panamensis_, occur only between genera. The need for caution in inferring the limits of variation for a particular structure in one species or genus, on the basis of variation in another group, is therefore obvious.