_B. m. musculus_ intergrades with _B. m. pallidus_ in eastern Michoacan and central and western Guerrero. Specimens from San Jose Prura and 12 mi. S Tzitzio, Michoacan, though referable to _B. m. musculus_ because of slightly larger size of crania are intermediate in size and color between the smaller and slightly darker _pallidus_ to the south and east and the larger, slightly paler _musculus_ to the northwest. _Specimens examined._--Total 156 all from the Republic of Mexico, and distributed as follows: NAYARIT: 3 mi. NNW Las Varas, 150 ft., 1. JALISCO: 7 mi. W Ameca, 4000 ft., 2[10]; _6 mi. W Ameca_, 4300 ft., 3[10]; _10 mi. S Ameca_, 5800 ft., 1[10]; _13 mi. S, 15 mi. W Guadalajara_, 3; _13 mi. S, 9-1/2 mi. W Guadalajara_, 1; _3 mi. ENE Santa Cruz de las Flores_, 1; 27 mi. S, 12 mi. W Guadalajara, 1; _4 mi. NE Autlan_, 3000 ft., 5[10]; _Sierra de Autlan_, 5000 ft., 2[10]; _2-1/2 mi. NNE Autlan_, 3000 ft., 8; 2 mi. SSE Autlan, 1; _5 mi. S Purificacion_, 2; Chamela Bay, 1[10]; _2 mi. N La Resolana_, 1500 ft., 6[10]; _1 mi. N San Gabriel_, 4000 ft., 32[10]; 2 mi. N Cuidad Guzman, 5000 ft., 1; 3 mi. E Navidad, 4300 ft., 10[10]. COLIMA: _type locality_, 10[11] (including the type); _3 mi. SE Colima_ (_City_), 5[10]; _4 mi. SW Colima City_, 1; Armeria, 200 ft., 8[11]; _Paso del Rio_, 20[10]. MICHOACAN: 12 mi. S Tzitzio, 6[10]; San Jose Prura, 4[12]; 1 mi. E, 6 mi. S Tacambaro, 4000 ft., 3[13]; La Salada, 3[11]; 1/2 mi. SE Coalcoman, 15[10]. _Marginal records._--NAYARIT: 3 mi. NNW Las Varas, 150 ft. JALISCO: 3 mi. E Navidad, 4300 ft.; 27 mi. S, 12 mi. W Guadalajara. MICHOACAN: 12 mi. S Tzitzio; San Jose Prura; 1/2 mi. SE Coalcoman. COLIMA: Armeria, 200 ft. JALISCO: Chamela Bay. [10] Univ. Michigan, Museum of Zoology. [11] U. S. Nat. Museum (Biol. Surv. Coll.). [12] Chicago Natural History Museum. [13] Univ. California, Mus. Vert. Zoology. =Baiomys musculus nigrescens= (Osgood) _Peromyscus musculus nigrescens_ Osgood, Proc. Biol. Soc. Washington, 17:76, March 21, 1904; Elliot, Field Columb. Mus. Publ., 105(4):136, July 1, 1905; Lyon and Osgood, Bull. U. S. Nat. Mus., 62:135, January 15, 1909; Osgood, N. Amer. Fauna, 28:259, April 17, 1909. _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).