_Remarks._--Merriam (1892:170) described _Sitomys_ [= _Baiomys_] _musculus_ on the basis of 23 specimens (from Colima City, Colima; Armeria, Colima; Plantinar, and Zapotlan, Jalisco). According to the original description, _B. musculus_ resembled a small house mouse and was smaller than any known species of _Sitomys_ except _S. taylori_ [= _Baiomys taylori_]. From _taylori_, _musculus_ differed in being larger [in size of body], and in having longer ears and tail, and larger hind feet. When Allen and Chapman (1897:203) described _Peromyscus_ [= _Baiomys_] _musculus brunneus_ from Jalapa, Veracruz, the specimens described by Merriam from Colima and Jalisco became representative of the nominal subspecies _B. m. musculus_. Osgood (1909:258) assigned specimens from Colima, Guerrero, Jalisco, Michoacan, Morelos, Oaxaca, Puebla, Sinaloa, Veracruz, and Zacatecas to the subspecies _musculus_. Subsequently, Russell (1952:21) named the subspecies _pallidus_ from the arid lowlands of Morelos; Hooper (1952:96) described the subspecies _infernatis_ from northern Oaxaca and southeastern Puebla; and Goodwin (1959:1) described a new subspecies _nebulosus_ from the Oaxaca highlands. Each of the subspecies mentioned immediately above was described from within the geographic range assigned to _B. m. musculus_ by Osgood (_loc. cit._). Hall and Kelson (1959:661) mapped the range of _B. m. musculus_ so as to include Colima, parts of Jalisco, Michoacan, Guerrero, Oaxaca, and Veracruz. Lukens (1955:159), in a study of the mammals of Guerrero, has shown that the characters attributed to _B. m. pallidus_ are not significantly different from those of pygmy mice studied from Guerrero. He (_loc. cit._) concluded that: (1) if the specimens of pygmy mice from central Guerrero were typical of the subspecies _musculus_, then _pallidus_ did not deserve subspecific recognition, or; (2) the name _B. m. musculus_ should be restricted to the larger pygmy mice inhabiting the lowlands immediately adjacent to the Pacific Coast and the area to the north. My data (see Figure 12) show pygmy mice from southwestern Nayarit, northwestern and central Jalisco, Colima, and parts of Michoacan to be significantly larger in certain cranial and external measurements than pygmy mice from Guerrero, Oaxaca, Morelos, and parts of Puebla. This finding essentially corroborates Hooper's (1952a:96) findings. It seems advisable, therefore, to restrict the range of _B. musculus musculus_ to the large mice inhabiting west-central Mexico and the coastal lowlands of Colima and Michoacan. The name _pallidus_ is applicable to the smaller mice occupying Morelos, southwestern Puebla, Guerrero, Oaxaca, and southwestern Chiapas.