Remarks.—Burt (1938:54) reluctantly assigned specimens from Ciudad Obregón to B. t. paulus, probably being influenced by the resemblance in size. He suggested that, perhaps, a distinct subspecies occurs in the State of Sonora. Study of larger series of specimens than were available to Burt reveals that populations of pygmy mice inhabiting the northwest coastal plains of México are indeed distinct.
The darkest of the material assigned to canutus is from Nayarit (for specific localities see specimens examined). According to Tamayo (1949:Carta de Suelos), color of soil changes from chestnut in northern Sinaloa to black in southern Sinaloa and northern Nayarit. There seems, therefore, to be a close correlation between color of pelage and color of soil in this area. In Nayarit, particularly in the central and southern parts, the mice are intermediate in color between the paler, grayer population to the north and the more brownish samples, representative of allex to the south. The coastal vegetation changes from the arid tropical thorn forests of the north and central parts of Sinaloa to a savannah in Nayarit, thence to a tropical deciduous forest farther south (see Leopold, 1950:508).
In size and color, specimens from 3 mi. SE Tepic and 2 mi. SW Rosa Morada are intermediate between the larger, grayer canutus and the smaller, light-brownish allex. In size of cranium, these specimens are more nearly like canutus, and are referred to that subspecies. Mice from the western coastal plain are relatively homogeneous as regards size of body and skull, except that those from 13.5 mi. S Acaponéta, Nayarit, average somewhat larger.
B. t. canutus, like B. t. subater, is predominantly a lowland or coastal subspecies. The pallor of the former, that lives on generally paler soils, presumably is of adaptive value.
Pygmy mice are seemingly rare in the northern part of the range of this subspecies. J. Raymond Alcorn and Albert Alcorn were successful in collecting only two specimens from the type locality after three successive nights of trapping with 100 traps set each night. Only six specimens are known from Sonora. These were obtained in the irrigated regions of Ciudad, Obregón, and Navajoa. Charles Sibley obtained one specimen 10.6 mi. SE Ciudad Obregón in a "maguey field." I obtained one specimen 1 mi. NNW Navajoa in a sparse grassway, 20 feet wide, bordering an open sewer, which coursed northward into the Río Mayo. Irrigated wheat fields bordered the grassway and ditch.
Specimens examined.—Total 70 all from the Republic of México and distributed as follows: Sonora: [Ciudad] Obregón, 4[44]; 10.6 mi. SE [Ciudad] Obregón, 1[45]; 1 mi. NNW Navajoa, 1. Sinaloa: type locality, 2 (including the type); Culiacán, 175 ft., 2[46]; Mazatlán, 1[48]; 15 mi. N Rosario, Chelé, 300 ft., 35[47]; Rosario, 3[46]; Escuinapa, 5[48]; Railroad Station Escuinapa, 43 ft., 2[45]. Nayarit: Acaponéta, 4[46]; 13.5 mi. S Acaponéta Junction, 6[49]; 2 mi. SW Rosa Morada, 2; 2 mi. WNW Tepic, 3200 ft., 1; 3 mi. SE Tepic, 1.
Marginal records.—Sonora [Ciudad] Obregón. Sinaloa: type locality; Escuinapa. Nayarit: Acaponéta; 3 mi. SE Tepic. Sinaloa: Mazatlán.
[44] Coll. Univ. California, Los Angeles.