_Comparisons._--From _B. t. taylori_, _B. t. fuliginatus_ differs in: dorsum slightly darker than in darkest _taylori_; tail densely haired, bicolored rather than unicolored; belly sooty to grayish rather than grayish to whitish; forefeet and hind feet sooty to grayish rather than flesh-colored; incisive foramina less bowed laterally, more nearly straight; interparietal compressed anteroposteriorly, less diamond-shaped. From _B. t. paulus_, _B. t. fuliginatus_ differs in: dorsum dusky to blackish rather than fawn color; belly sooty to grayish rather than buffy to whitish-gray; forefeet and hind feet sooty to grayish rather than whitish; zygoma more nearly forming a right angle with rostrum or skull, less tapered anteriorly; anterior part of jugal possessing ventral projection; jugal extending nearly to lacrimal on posterior surface of maxillary part of zygoma. From _B. t. analogous_, _B. t. fuliginatus_ differs in: mid-dorsal region blacker, less brownish; tail distinctly bicolored rather than unicolored to faintly bicolored; incisive foramina not constricted medially; presphenoid broader (at narrowest point); jugal differs much the same as it does from _paulus_; nasals anteriorly truncate instead of rounded. _Remarks._--Dalquest (1953:155-157) and Booth (1957:15) assigned all of the pygmy mice that they examined from the state of San Luis Potosi to _B. t. taylori_. Examination of all of the material that was available to Dalquest, plus additional specimens at the University of Kansas Museum of Natural History, reveals that there are three subspecies in San Luis Potosi. _B. t. taylori_ occurs in the eastern part of the State at lower altitudes; _B. t. analogous_ occurs to the southeast at higher altitudes; _B. t. fuliginatus_ occurs in the northeastern part of the State in the Sierra Madre Oriental.