18.0 (17.5-18.6) _15_, 9-1/2 mi. W New Mexico state line, Ariz. 18.9 (18.2-19.4) _6_, 7 mi. S. La Belle, Jefferson Co., Texas. 18.2 (17.8-18.5) _10_, San Antonio, Bexar Co., Texas. 18.2 (18.0-18.5) _5_, 2 mi. W Minaca, Chihuahua. 18.0 (17.6-19.0) _22_, 6 mi. SW San Geronimo, Coahuila. 18.2 (18.1-18.3) _3_, Ciudad Obregon, Sonora. 18.1 (17.4-18.5) _5_, vic. (see p. 649) Durango, Durango. 18.1 (17.5-18.5) _9_, Jaumave, Tamaulipas. 18.2 (17.7-18.9) _19_, 15 mi. N Rosario Chele, Sinaloa. 17.9 (17.4-18.3) _27_, vic. (see p. 655) Altamira, Tamaulipas. 18.3 (17.9-18.7) _9_, Valparaiso, Zacatecas. 18.1 (18.1-18.2) _4_, Ciudad del Maiz, San Luis Potosi. 18.6 (18.3-18.9) _8_, Tepic, Nayarit. 18.0 (17.7-18.4) _18_, 4 mi. N, 5 mi. W Leon, Guanajuato. 18.1 (17.5-18.9) _28_, 6 mi. E Queretaro, Queretaro. 17.7 (17.1-18.1) _17_, 1 mi. SSE Ameca, Jalisco. 17.3 (16.8-17.9) _10_, 2 mi. SSE Autlan, Jalisco. 18.0 (17.5-18.6) _10_, 1 mi. S, 11 mi. W Zamora, Michoacan. 17.6 (17.4-18.2) _8_, Colima, Colima. _Baiomys musculus_ 20.2 (19.9-20.3) _6_, vic. (see p. 622) Ameca, Jalisco. 20.2 (19.9-20.3) _6_, 2 mi. SSE Autlan, Jalisco. 19.6 (19.2-20.1) _6_, Jalapa, Veracruz. 20.3 (19.7-20.9) _9_, Colima, Colima. 19.5 (19.0-20.0) _10_, Cerro Gordo, Veracruz. 19.8 (19.4-20.3) _6_, 6 mi. S Izucar de Matemores, Puebla. 20.0 (18.8-20.5) _7_, Teotitlan, Oaxaca. 20.1 (19.7-20.7) _7_, 1 km. NW Chapa, Guerrero. 19.9 (19.4-20.4) _8_, 5 mi. ESE Tecpan, Guerrero. 19.5 (19.1-20.1) _22_, 3 mi. ESE Oaxaca, Oaxaca. 19.5 (19.1-19.9) _11_, Valley of Comitan, Chiapas. 18.9 (18.2-20.1) _17_, Tehuantepec, Oaxaca. 18.9 (18.4-19.7) _15_, 6 mi. NW Tonala, Chiapas. 19.1 (18.8-20.4) _10_, 1 mi. S Rabinal, Guatemala. 19.7 (18.8-20.4) _10_, Lake Amatitlan, Guatemala. 19.2 (18.4-19.8) _26_, vic. (see p. 625) San Salvador, El Salvador. 19.3 (18.9-19.9) _24_, 8 mi. S Condega, Esteli, Nicaragua.] CONCLUSIONS 1. Two Recent species, each polytypic with eight subspecies, and five fossil species are recognized. 2. The phyletic trends in the genus _Baiomys_ have been from an ancestral stock that possessed relatively brachydont teeth having raised cingular ridges and orthodont to prooedont incisors, to species having hypsodont teeth with reduced cingular ridges and retrodont incisors. 3. Reduction of cingular ridges in pygmy mice is associated with an existence in open grassland (more xeric than mesic), whereas, the presence of cingular ridges is associated with an existence in a savannah habitat (more mesic than xeric). 4. Shifts of geographical range of populations of pygmy mice at and near the periphery of their geographic range may account for the differentiation of the extinct species.