Female (based on ten adults from Sierra Nevada of California): See measurements and plates 33-35; weight, 2.2 (1.8-2.4) grams; basilar length, 38.2 (36.7-39.5); zygomatic breadth more (except in one specimen) than distance between condylar foramen and M1 and more (save in two specimens) than distance between anterior palatine foramen and anterior margin of tympanic bulla; postorbital breadth less than length of upper premolars and less than (except in one specimen) width of basioccipital measured from medial margin of one foramen lacerum posterior to its opposite; least width of palate more or less than either outside or inside length of P4 but generally less than inside length; tympanic bulla as far posterior to foramen ovale as width of 3 to 5-1/2 upper incisors; height of tympanic bulla more or less (usually more) than distance from its anterior margin to foramen ovale; length of tympanic bulla more than length of lower molar and premolar tooth-row and more or less than length of rostrum. The skull of the female averages 41 per cent lighter than that of the average male. Compared with the skull of _M. f. longicauda_, that of both sexes averages smaller in every measurement taken. Males of _nevadensis_, on the average, relative to the basilar length, are narrower in the interorbital region and across the zygomata but have the orbitonasal length greater. Stated in another way, the rostrum of _longicauda_ appears to be shorter and broader and the zygomata are more expanded. Females of _nevadensis_, on the average, relative to the basilar length are narrower across the mastoid processes and zygomata and have the braincase deeper at the anterior margin of the basioccipital. Also in _nevadensis_ the mastoid processes do not project so far laterally beyond the braincase, the lambdoidal crest and postorbital processes are less well developed and except in the interparietal region, the temporal ridges hardly meet and they form a sagittal furrow rather than a low sagittal crest which characterizes adult females of _longicauda_. Each of these differences separating the females of _longicauda_ from those of _nevadensis_ are of the same nature, although not necessarily of the same degree, as those which appear in _longicauda_ with increasing age. The differences mentioned above are readily appreciable when series of specimens are compared. However, none of the differences is of great degree, and most parts of the skulls of the two subspecies are of similar relative proportions. Even so, there is but little overlap in actual size. Comparisons with the skulls of _M. f. oribasus_, _alleni_, _neomexicana_, _arizonensis_, _inyoensis_, _pulchra_, _xanthogenys_, _munda_, _saturata_, _oregonensis_, _washingtoni_, _altifrontalis_, and _effera_ are made in the accounts of those subspecies.