_Baiomys musculus musculus_, Miller, Bull. U. S. Nat. Mus., 79:137, December 31, 1912 (part); Miller, Bull. U. S. Nat. Mus., 128:318, April 29, 1924 (part); Davis, Jour. Mamm., 25:394, December 12, 1944 (part); Hooper, Jour. Mamm., 28:50, February 15, 1947 (part); Goldman, Smith, Miscl. Coll., 115:336, July 31, 1951 (part); Miller and Kellogg, Bull. U. S. Nat. Mus., 205:512, March 3, 1955 (part); Booth, Walla Walla Publs., Dept. Biol. Sci., 20:15, July 10, 1957 (part); Hall and Kelson, The Mammals of North America, 2:661, March 31, 1959 (part); Goodwin, Amer. Mus. Novitates, 1929:1, March 5, 1959. _B._ [_aiomys_] _m._ [_usculus_] _musculus_, Hooper, Jour. Mamm., 33:97, February 18, 1952 (part). _B._ [_aiomys_] _m._ [_usculus_] _nigrescens_, Hooper, Jour. Mamm., 33:97, February 18, 1952 (part). _Baiomys musculus nebulosus Goodwin_, Amer. Mus. Novitates, 1929, March 5, 1959. _Type._--Adult female, skin and skull; No. 4501 Texas A&M Cooperative Wildlife Collection; 12 kms. NW Axochiapan, 3500 feet, Morelos, Republic of Mexico, obtained on July 28, 1950, by W. B. Davis, original number 5112. _Range._--Guerrero thence eastward into Morelos and west central Puebla along the southern edge of the Transverse Volcanic Biotic Province (Goldman and Moore, 1945:349), south into Oaxaca, see Figure 10. Zonal range: largely Arid Lower Tropical Subzone of Goldman (1951:330). Occurs from near sea level in Oaxaca and Guerrero up to 6550 feet in Oaxaca. _Diagnosis._--Size medium for the species; dorsum Buffy Brown in palest series to Olive-Brown in darkest series, individual hairs Warm Buff, Neutral Gray basally, some with black tips and a subterminal band of Warm Buff, guard hairs of dorsum black-tipped, gray basally; hairs on sides creamy-buff, gray basally; face same color as back fading to white on throat; vibrissae white-tipped, pale brown basally; venter, whitish with tinges of buff on lower throat, individual hairs having tips white to buffy-white, light gray basally; dorsal surface of forefeet and hind feet whitish to flesh-color; tail indistinctly bicolored, brownish above, grayish brown below; zygoma bowed as in _B. m. grisescens_; tail short; average and extreme external and cranial measurements for 17 adults from Tehuantepec, Oaxaca, are: total length, 117.3 (110-126); length of tail vertebrae, 46.9 (41-51); length of body, 70.4 (65-76); length of hind foot, 15.8 (15-16); occipitonasal length, 18.9 (18.2-20.1); zygomatic breadth, 10.1 (9.7-10.6); postpalatal length, 6.9 (6.6-7.5); least interorbital breadth, 3.8 (3.6-3.9); length of incisive foramina, 4.4 (4.2-4.7); length of rostrum, 6.7 (6.3-7.2); breadth of braincase, 9.3 (8.7-9.7); depth of cranium, 6.6 (6.4-6.8); alveolar length of maxillary tooth-row, 3.2 (3.1-3.4); for photographs of skull, see Plate 1g, and Plate 3g.