Speaking now of full species, the most inflated tympanic bullae in American weasels are in _Mustela frenata_, and more restrictedly in those subspecies of it which occur in the temperate region. Subspecies of _M. frenata_ in Central and South America, as already noted, have less inflated bullae. The tropical weasel, _Mustela africana_, of the Amazon drainage of South America has the bullae still less inflated (see fig. _i_ of pl. 39 and fig. _f_ of pl. 40). The bullae are less inflated even than in the mink, subgenus _Lutreola_. In _M. africana_ the cleidomastoideus, omotrachelian, levator scapulae, and rhomboideus profundus muscles take origin from a fossa on the mastoid bone, whereas in the forms with greatly inflated bullae these muscles take origin from a raised ridge or tubercle. Using _Mustela frenata_ of the temperate region as a starting point and proceeding northward, a reduction in inflation of the tympanic bulla is seen also in that direction in that _Mustela erminea_ has less inflated bullae. The bullae are less inflated in southern than in far northern (arctic) populations of _Mustela erminea_. In _erminea_ the lesser inflation is real enough but at the same time there appears to be less inflation than actually exists, for the squamosal floor of the braincase is "pushed down." This places the anterior end of the tympanic bulla farther in the braincase than it otherwise would be. Although the anterior end of the bulla is flattened to the extent that it resembles the sharp edge of a splitting-wedge, inspection of the lateral and medial edges shows that in its central part the bulla is more inflated than it is in the weasels of Central and South America. For reasons set forth later, _M. erminea_ is judged to resemble the ancestral stem form more closely than does any one of the other three American species of weasels. If this judgment is correct, the shape of the tympanic bullae of the American weasels may be explained as follows: In the subspecies of _Mustela frenata_ of the temperate regions of North America the bullae have most nearly been pushed out of the braincase and at the same time have undergone some enlargement. The subspecies of this same species in Central and South America represent an earlier stage in the evolution of American weasels and retain less inflated bullae--less inflated even than those of the southern subspecies of _erminea_. _M. africana_ probably separated from the stem form at a still earlier time if we may judge by the lesser inflation of its tympanic bullae. There are other reasons for thinking that _africana_ separated from the stem form earlier than _M. frenata_ did. During the time that elapsed since the separation of _M. frenata_ from the stem form, the tympanic bullae of _M. erminea_ probably increased slightly in size, as probably also did the brain but without shoving the auditory complex forward from its former position. DISTRIBUTION AND SPECIATION