_Comparisons._--For comparisons with _B. m. nigrescens_ and _B. m. brunneus_, see accounts of those subspecies. From _B. m. pallidus_, _B. m. infernatis_ differs in: sides, ears, and dorsum paler (less of dark brown); venter whitish gray rather than gray with tinge of buff and brown; forefeet and hind feet paler; tail bicolored, not unicolored; incisive foramina longer and not constricted posteriorly; mastoid process turning dorsally and sickle-shaped at posteriormost point rather than capitate. _Remarks._--_B. m. infernatis_ resembles _B. m. handleyi_ more than any other subspecies in color of pelage and in external and cranial dimensions. The resemblance in color between _B. m. pallidus_, in certain parts of its range, and _B. m. handleyi_ may have resulted from nearly parallel selective forces that gave rise to two subspecies, widely separated geographically. The same relation obtains between _B. m. infernatis_ and _B. m. handleyi_. Both inhabit arid river basins. In them, pale soil and low relative humidity are important passive factors of selection that give adaptive value to the pale colors of pelage of both _infernatis_ and _handleyi_. Specimens from 6-1/2 mi. SW Izucar de Matemores, and 1 mi. SSW Tilapa, Puebla, are intergrades between _B. m. infernatis_ and _B. m. pallidus_. These specimens are intermediate in color and cranial characters between the aforementioned subspecies but possess more of the pale brown overtones seen in paratypes of _pallidus_, and are best referred to that subspecies. _Specimens examined_ (All in Univ. Michigan, Mus. Zool.).--Total 18, all from the Republic of Mexico and distributed as follows: PUEBLA, Tepanaco, 6000 ft., 3, Tehuacan, 5400 ft., 3. OAXACA: Type locality, 3100 ft., 12 (including the type). _Marginal records._--See specimens examined. =Baiomys musculus musculus= (Merriam) _Sitomys musculus_ Merriam, Proc. Biol. Soc. Washington, 7:170, September 29, 1892; Lyon and Osgood, Bull. U. S. Nat. Mus., 62:135, January 15, 1909. _Baiomys musculus_ [= _musculus_], Mearns, Bull. U. S. Nat. Mus., 56:381, April 13, 1907; Hooper, Jour. Mamm., 36:29, May 26, 1955. _Peromyscus musculus_ [_musculus_], J. A. Allen and Chapman, Bull. Amer. Mus. Nat. Hist., 9:203, June 16, 1897; Elliot, Field Columb. Mus. Publ., 105(4):135, July 1, 1905; Osgood, N. Amer. Fauna, 28:257, April 17, 1909 (part). [_Peromyscus_] _musculus_, Trouessart, Cat. Mamm., 1:518, 1898. [_Peromyscus_] _musculus_ [_musculus_], Elliot, Field Columb. Mus. Publ., 95(4):175, July 15, 1904.