Capitalized color-terms refer to Ridgway (1912). Color terms without initial letters capitalized do not refer to any one standard. The names of the cusps and ridges of the teeth (see Figure 2) are those suggested by Wood and Wilson (1936:389-390). Terminology of the enamel grooves and folds is that of Hershkovitz (1944:17) and Hooper (1952b:20-21). Because secondary sexual variation was not significant (see page 597), both males and females of like age and pelage were used in comparisons of samples designed to reveal geographic variation. The species are arranged from less to more progressive; the subspecies are arranged alphabetically. In the synonymy of each subspecies, the plan has been to cite: (1) the name first proposed; (2) the first usage of the name combination employed by me; (3) all other name combinations in chronological order that have been applied to the subspecies concerned. The localities of specimens examined are listed by country from north to south. Within a country, the listing is by state, beginning with the northwesternmost state and proceeding by tiers (west to east) to the southeasternmost state. Within a state of the United States, the listing is by counties in the same geographic order as described for states. Within any county in the United States, within any state in Mexico, and within any country in Central America, the listing of localities is from north to south. When more than one locality is on the same line of latitude, the westernmost locality is listed first. Marginal localities for each subspecies are listed in a paragraph at the end of each account. Each marginal locality is mapped by means of a circle. The circles are listed in clockwise order, beginning with the northernmost. When more than one of these localities lies on the same line of latitude, the westernmost is cited first. Localities not represented on the distribution maps, so as to avoid undue crowding of symbols, are italicized in the lists of specimens examined. [Illustration: FIG. 2. Occlusal views of molars. x 13. A. _B. taylori analogous_, subadult, female, No. 28102 KU, 4 km. ENE Tlalmanalco, 2290 meters, Estado de Mexico. Right, upper molars. B. _B. musculus musculus_, subadult, male, No. 45456 USNM, Colima, Colima, Mexico. Left, upper molars. A'. _B. taylori analogous_, subadult, female, No. 28102 KU 4 km. ENE Tlalmanalco, 2290 meters, Estado de Mexico. Left, lower molars. B'. _B. musculus musculus_, subadult, male, No. 45456 USNM, Colima, Colima, Mexico. Right, lower molars.] The largest single collection of pygmy mice is in the University of Kansas Museum of Natural History, and, unless otherwise indicated, specimens cited in the taxonomic accounts beyond are there. I am indebted to the following named institutions and persons for making specimens available for study: American Museum of Natural History, G. G. Goodwin and R. G. VanGelder.