_Comparisons._--For comparisons with _B. m. brunneus_ and _B. m. infernatis_, see accounts of those subspecies. From _B. m. musculus_, _B. m. pallidus_ differs in: dorsum more olive-gray and brown, less ochraceous on either side of mid-dorsal region; face below eye grayish, not buffy; sides gray with buffy overtone, not creamy with light yellow overtones; venter grayish-white rather than an olive-buff; zygomata more tapering anteriorly; maxillary part of zygoma narrower when viewed from above; external and cranial dimensions smaller. From _B. m. nigrescens_, _B. m. pallidus_ differs in: dorsum paler, fewer black hairs medially; face paler, less sooty; vibrissae brownish with white tips rather than black with brownish tips; venter paler; dorsal surface of forefeet and hind feet whitish to flesh-colored rather than sooty to dusky-white; tail paler; nasals slightly more attenuated; averaging slightly larger in external and cranial measurements. _Remarks._--Russell (1952:21) described _pallidus_, on the basis of specimens from the arid Balsas Basin, of Morelos, as pale gray dorsally. After examining the original material from Morelos, I find the dorsal color of _pallidus_ to be much closer to a buffy brown than a pale grayish. Even so, smaller size differentiates _pallidus_ from _musculus_. _B. m. infernatis_, not _B. m. pallidus_, is the most pallid of all named subspecies of _B. musculus_. _B. m. pallidus_ intergrades to the northwest with _B. m. musculus_, to the northeast with _B. m. infernatis_, and to the southeast with _B. m. nigrescens_. According to Goodwin (1959:2), _B. m. nebulosus_ (named on the basis of one specimen) differs from _B. m. musculus_ [= _pallidus_] from southern Oaxaca in: darker and longer pelage; larger skull; interorbital region broader and less constricted posteriorly. From _B. m. nigrescens_ and _B. m. brunneus_, _B. m. nebulosus_ differs as follows: pelage longer and softer; skull larger. Study of specimens of _B. musculus_ from Oaxaca reveals considerable variation in external and cranial measurements as well as color, corresponding to that reported by Goodwin (_loc. cit._). Specimens from higher altitudes average somewhat darker and larger in external and cranial size than those at lower elevations. These differences seem to be microgeographic and not of subspecific rank. Among specimens that I have studied in Oaxaca are several from different localities (KU 63052, an adult male, from 3 mi. W Miahuatlan; KU 68964, an adult male from 3 mi. W Mitla, 6000 ft.; KU 63055, an adult female from 3 mi. S Candelario, 1200 ft.) that, according to Goodwin (_in. litt._) match _nebulosus_ in reported color, size of body and skull (except for the region of the rostrum).