Fig. 12. Averages of the occipitonasal lengths of skulls of adults at 19 localities of occurrence (solid symbols) of Baiomys taylori, and at 17 localities of occurrence (open symbols) of Baiomys musculus. Note that the occipitonasal length decreases from north to south in each of the two species, and that in the region where the two species occur together, west-central México, B. taylori is smallest and B. musculus is largest. Average, extremes, number of specimens averaged (in italic type), and name of locality, from north to south for each species, are as follows:

Baiomys taylori

18.0 (17.5-18.6) 15, 91/2 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 Miñaca, Chihuahua.
18.0 (17.6-19.0) 22, 6 mi. SW San Gerónimo, Coahuila.
18.2 (18.1-18.3) 3, Ciudad Obregón, Sonora.
18.1 (17.4-18.5) 5, vic. (see [p. 649]) Durango, Durango.
18.1 (17.5-18.5) 9, Jaumavé, Tamaulipas.
18.2 (17.7-18.9) 19, 15 mi. N Rosario Chelé, Sinaloa.
17.9 (17.4-18.3) 27, vic. (see [p. 655]) Altamira, Tamaulipas.
18.3 (17.9-18.7) 9, Valparaíso, Zacatecas.
18.1 (18.1-18.2) 4, Ciudad del Maíz, San Luis Potosí.
18.6 (18.3-18.9) 8, Tepic, Nayarit.
18.0 (17.7-18.4) 18, 4 mi. N, 5 mi. W León, Guanajuato.
18.1 (17.5-18.9) 28, 6 mi. E Querétaro, Querétaro.
17.7 (17.1-18.1) 17, 1 mi. SSE Ameca, Jalisco.
17.3 (16.8-17.9) 10, 2 mi. SSE Autlán, Jalisco.
18.0 (17.5-18.6) 10, 1 mi. S, 11 mi. W Zamora, Michoacán.
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 Autlán, 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 Izucár de Matemores, Puebla.
20.0 (18.8-20.5) 7, Teotitlán, Oaxaca.
20.1 (19.7-20.7) 7, 1 km. NW Chapa, Guerrero.
19.9 (19.4-20.4) 8, 5 mi. ESE Tecpán, Guerrero.
19.5 (19.1-20.1) 22, 3 mi. ESE Oaxaca, Oaxaca.
19.5 (19.1-19.9) 11, Valley of Comitán, Chiapas.
18.9 (18.2-20.1) 17, Tehuantepec, Oaxaca.
18.9 (18.4-19.7) 15, 6 mi. NW Tonalá, Chiapas.
19.1 (18.8-20.4) 10, 1 mi. S Rabinal, Guatemala.
19.7 (18.8-20.4) 10, Lake Amatitlán, 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, Estelí, 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 proödont 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.

5. The two living species, B. musculus and B. taylori, are seemingly derived from a common ancestor that in morphological structure was intermediate between B. minimus and B. musculus.