_Characters of the species._--Size large (extremes in external measurements of adults; total length, 100-135; length of tail vertebrae, 33-56; length of hind foot, 14.1-17; length of ear, 9-12); upper parts dark reddish brown, or ochraceous-buff to nearly black; underparts pale pinkish buff to white or pale buffy. _Geographic variation._--Eight subspecies are here recognized (see Figure 10). Features that vary geographically are external size, color of pelage, certain cranial dimensions (occipitonasal length, zygomatic breadth, least interorbital breadth, length of rostrum, length of incisive foramina, depth and breadth of cranium, and alveolar length of upper molar tooth-row). External and cranial size (except for _B. m. handleyi_) is less in the southernmost subspecies, _B. m. pullus_, _B. m. grisescens_, _B. m. nigrescens_, and more in the northernmost subspecies, _B. m. musculus_, _B. m. brunneus_, and _B. m. infernatis_. Increase in size from south to north is in keeping with Bergman's Rule that within a species, smaller individuals occur in warmer parts of its geographic range. Southern pygmy mice at high altitudes average larger than those from low elevations, except where the two species are sympatric. There the Southern Pygmy Mouse is uniformly larger, regardless of altitude. Osgood (1909:257, 259) suggested that degree of relative humidity might in some way control color of pelage in both _B. taylori_ and _B. musculus_. In _B. musculus_, the darker subspecies, _B. m. brunneus_, _B. m. nigrescens_, and _B. m. pullus_, occur in zones of rather constant high relative humidity, whereas the paler subspecies _infernatis_, _musculus_, _handleyi_, and to a less extent _grisescens_ and _pallidus_, occur in zones of lower relative humidity. This is in keeping with Gloger's Rule, which states that melanins increase in the warm and humid parts of the range of a species, and reddish or yellowish-brown phaeomelanins prevail in arid climates. _B. m. musculus_ ranges into areas where relative humidity is such that darker pelages might be expected, but this is in the area where the two species are sympatric, and color of pelage may be an important character of recognition. [Illustration: FIG. 10. Distribution of _Baiomys musculus_. Known localities of occurrence are represented by circles and black dots; the former denote localities that are peripheral (marginal) for the subspecies concerned. 1. _B. m. brunneus_ 2. _B. m. grisescens_ 3. _B. m. handleyi_ 4. _B. m. infernatis_ 5. _B. m. musculus_ 6. _B. m. nigrescens_ 7. _B. m. pallidus_ 8. _B. m. pullus_] _Natural History_