Generally speaking, populations of _C. castanops_ from northeastern Coahuila are related, as a group, in color and cranial features. Partial isolation of subspecies in this area results chiefly from discontinuity of suitable soils rather than from topography. These pocket gophers occur most commonly in the deep, sandy soils which are found along streams, especially where farm lands are irrigated. In western and southern Coahuila, mountains extending in both north-south and east-west directions act as partial barriers to the passage of _C. castanops_. Within this large area, pocket gophers occur in desert basins many of which are enclosed on two or more sides by mountains. Even so, with the exception of the smaller _C. c. consitus_ of northwestern Coahuila, all known subspecies occurring at lower elevations in the western and southern part of the state show close relationships in color and cranial features. Those subspecies in the higher parts of southeastern Coahuila by their small size and dark color reflect to a high degree their isolation in an elevated habitat. Males of _C. castanops_ differ greatly from females of equal age; consequently animals of the same sex, as well as of the same age, are used herein for taxonomic comparisons. Since, of any given age-group, females show less individual variation than do males, we have relied more on the characteristics of the females in this taxonomic study. Only specimens taken at approximately the same times of the year have been compared for color of pelage. Capitalized color terms are those of Ridgway, Color Standards and Color Nomenclature, Washington, D. C., 1912. Specimens made available through the courtesy of the authorities of the Biological Surveys Collection of the United States National Museum are indicated in the accounts of subspecies as BSC; other specimens listed are in the collection of the University of Kansas Museum of Natural History. Assistance with field work is acknowledged from the Kansas University Endowment Association and the National Science Foundation. In any one of the lists of "Specimens examined" beyond, the order of arrangement of the localities is from north to south. Those localities listed in Roman type are represented on the distribution map (Figure 1) by blacked-in circles. Each of several circles covers two or more localities because the localities are close together. In any such instance the northernmost place is listed in Roman type and the names of the other places follow in Italic type. Measurements in millimeters are given in table 1 for females and in table 2 for males. +Cratogeomys castanops convexus+ Nelson and Goldman 1934. _Cratogeomys castanops convexus_ Nelson and Goldman, Proc. Biol. Soc. Washington, 47:142, June 13, type from 7 mi. E Las Vacas [= Villa Acuña], Río Grande Valley, Coahuila (opposite Del Río, Texas). _Distribution._--Extreme northern Coahuila, east and north of the Serranías del Burro (see fig. 1).