Baiomys musculus nigrescens, Miller, Bull. U. S. Nat. Mus., 79:137, March 31, 1912; Miller, Bull. U. S. Nat. Mus., 128:318, April 29, 1924; Goodwin, Bull. Amer. Mus. Nat. Hist., 68(1):40, December 12, 1934; Ellerman, The Families and Genera of Living Rodents, 2:402, March 21, 1941; Poole and Schantz, Bull. U. S. Nat. Mus., 178:259, March 6, 1942; Hooper, Jour. Mamm., 28:50, February 15, 1947; Goldman, Smith. Miscl. Coll., 115:357, July 31, 1951; Miller and Kellogg, Bull. U. S. Nat. Mus., 205:513, March 3, 1955; Booth, Walla Walla Publs., Dept. Biol. Sci., 20:15, July 10, 1957; Hall and Kelson, The Mammals of North America, 2:661, March 31, 1959 (part).
[Peromyscus musculus] nigrescens, Elliot, Field Columb. Mus. Publ., 95(4):176, 1904.
B. [aiomys] m. [usculus] nigrescens, Goodwin, Bull. Amer. Mus. Nat. Hist., 79(2):160, May 29, 1942; Hooper, Jour. Mamm., 33:97, February 18, 1952 (part); Packard, Univ. Kansas Publs., Mus. Nat. Hist., 9:399, December 19, 1958.
B. [aiomys] m. [usculus] musculus, Booth, Walla Walla Publs., Dept. Biol. Sci., 20:15, July 10, 1957 (part).
Type.—Adult female, skin and skull; No. 76827 U. S. Nat. Mus. (Biol. Surv. Coll.); Valley of Comitán, Chiapas, Republic of México, obtained on December 9, 1895, by E. W. Nelson and E. A. Goldman, original number 8719.
Range.—Southern coastal region and eastern parts of Chiapas, southeastward into central and southern Guatemala, thence south into El Salvador (see [Figure 10]). Zonal range: parts of Lower Austral; also occurs in parts of the arid division of the Upper Tropical Life-zone, and in parts of the arid division of the Lower Tropical Life-zone; approximates a part of the Chiapas Highlands Biotic Province of Goldman and Moore (1945:349), and parts of the Guatemalan Subregion of Smith (1949:235).
Diagnosis.—Size medium to small for the species; dorsum Vandyke Brown mixed with blackish, individual hairs black-tipped with a subterminal band of Warm Buff, Neutral Gray at base; guard hairs of dorsum black distally, Neutral Gray basally; hairs on sides grayish-brown, facial region like dorsum; chin buffy-brown; vibrissae brown, ventrally some white; venter creamy-buff to grayish, individual hairs creamy-buff at tips, gray basally; in region of throat and chin, hairs tipped with Ochraceous-Buff; dorsal surface of forefeet and hind feet dull whitish gray to brownish-black; tail indistinctly bicolored, dusky above, grayish to brownish below; incisive foramina short, wide medially; [Pg 624] average and extreme external and cranial measurements of 15 adults from 6 mi. NW Tonalá, Chiapas, are as follows: total length, 107.5 (100-116); length of tail vertebrae, 41.1 (33-48); length of body, 66.1 (62-73); length of hind foot, 15.0 (14-16); length of ear, 10.9 (10-12); occipitonasal length, 18.9 (18.4-19.7); zygomatic breadth, 9.8 (9.4-10.2); postpalatal length, 6.9 (6.6-7.4); least interorbital breadth, 3.7 (3.5-3.8); length of incisive foramina, 4.4 (4.1-4.8); length of rostrum, 6.7 (6.1-7.1); breadth of braincase, 9.2 (9.0-9.4); depth of cranium, 6.9 (6.5-7.3); alveolar length of maxillary tooth-row, 3.1 (2.9-3.2); for photographs of skull, see [Plate 1f], and [Plate 3f].
Comparisons.—For comparisons with B. m. handleyi, B. m. grisescens, B. m. musculus, B. m. pallidus, and B. m. pullus, see accounts of those subspecies.
From B. m. brunneus, B. m. nigrescens differs in: dorsum blackish-brown rather than reddish to ochraceous brown; face and ears brownish-black rather than brownish with tinges of ochraceous; vibrissae darker; forefeet and hind feet darker; venter with more grayish tones; dorsalmost part of zygomatic plate projects farther anteriorly; interparietal oval to diamond-shaped and narrower anteroposteriorly; zygomata narrower at anteriormost part; slightly smaller in most cranial and external measurements.
From B. m. infernatis, B. m. nigrescens differs in: dorsum darker; region of face and ears darker; venter buffy to gray rather than whitish-buff; vibrissae darker; forefeet and hind feet darker; tail darker above and below; incisive foramina shorter, more constricted laterally; cranium slightly smaller in most dimensions.