_Comparisons._--From _B. t. ater_, _B. t. canutus_ differs in: dorsum slightly grayer; belly whitish to pale-gray with only faint tones of buff, rather than cinnamon-buff to buff-gray; forefeet and hind feet flesh-colored to grayish above instead of whitish to flesh-colored; tail paler above, less hairy, scales more evident; interparietal relatively larger from anteriormost to posteriormost points; incisive foramina tapering less abruptly posteriorly, not constricted towards midline; over-all size of body and cranium somewhat larger. From _B. t. paulus_, _B. t. canutus_ differs in: dorsum grayish-brown rather than fawn-colored (not differing appreciably from extremes of darker brown specimens of _paulus_); forefeet and hind feet flesh-colored to grayish above rather than white above; tail less hairy, unicolored to faintly bicolored rather than distinctly bicolored; braincase slightly larger; alveolar length of maxillary tooth-row slightly less. From _B. t. analogous_, _B. t. canutus_ differs in: dorsum paler, less of dark brown hues; belly paler; forefeet and hind feet slightly paler, less sooty above; tail less hairy, paler and having scales evident; jugal of zygoma extending ventrally to a point immediately above, instead of below, level of alveolus of upper molars; nasals more nearly truncate anteriorly; infraorbital foramina less deeply notched toward midline of skull; body and skull averaging smaller throughout. From _B. t. allex_, _B. t. canutus_ differs in: dorsal ground color grayish rather than fawn color having grayish overtones; underfur on dorsum darker gray; dorsal surface of forefeet and hind feet flesh-colored to grayish rather than flesh-colored; incisive foramina tapering to a point posteriorly rather than rounded posteriorly; interparietal relatively smaller; body and skull averaging larger throughout. _Remarks._--Burt (1938:54) reluctantly assigned specimens from Ciudad Obregon to _B. t. paulus_, probably being influenced by the resemblance in size. He suggested that, perhaps, a distinct subspecies occurs in the State of Sonora. Study of larger series of specimens than were available to Burt reveals that populations of pygmy mice inhabiting the northwest coastal plains of Mexico are indeed distinct.