Diagnosis.—Size medium for the species; dorsum Drab, terminal parts of individual guard hairs black, Neutral Gray basally, distal parts of underfur Pinkish Buff, proximally Neutral Gray; sides same color as dorsum; hairs in region of throat and chin white to base; venter whitish to Neutral Gray with tinges of Pinkish Buff; dorsal parts of forefeet and hind feet whitish with flesh-colored undertones, ventral parts whitish to dusky-gray; tail bicolored, grayish-brown above, white below; tip of tail not bicolored, instead grayish-brown throughout; ears pale brown, sparsely haired; incisive foramina long, not constricted posteriorly. Average and extreme external measurements for 9 adults from the type locality are as follows: total length, 113.9 (106-122); length of tail vertebrae, 44.1 (41-48); length of body, 71.0 (65-79); length of hind foot, 14.8 (13-16); length of ear, 11.9 (11-12). Average and extreme cranial measurements of 7 adults from the type locality are as follows: Occipitonasal length, 20.1 (19.7-20.4); zygomatic breadth, 10.4 (10.2-10.6); postpalatal length, 7.3 (7.0-7.7); least interorbital breadth, 4.2 (4.0-4.4); length of incisive foramina, 4.8 (4.4-5.6); length of rostrum, 7.2 (6.6-7.5); breadth of braincase, 9.6 (9.5-9.8); depth of cranium, 7.4 (7.1-7.6); alveolar length of maxillary tooth-row, 3.3 (3.1-3.4); for photographs of skull, see [Plate 1d], and [Plate 3d].

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 61/2 mi. SW Izucár 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 México and distributed as follows: Puebla, Tepanaco, 6000 ft., 3, Tehuacán, 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).