_Remarks._--_Cratogeomys castanops sordidulus_ is limited to the Llano de Ocampo, an elevated, desert plain surrounded on three sides, west, south and east, by higher mountainous country which seems to bar the passage of this rodent. On the eastern side this barrier extends north to the very banks of the Río Grande in the Cañon de Boquillas. This subspecies, therefore, is in contact with other populations of _Cratogeomys_ only to the north and northwest. This subspecies is well characterized by size, color and cranial characteristics. _Cratogeomys castanops sordidulus_ is not abundant; groups of mounds constructed by one or a few individuals were found in widely separated places. Mounds were often small, appeared old and, in other ways, were inconspicuous on arid flats. The animals were taken at elevations as low as 3250 feet and as high as 4150 feet. _Specimens examined._--Total, 13, from: 50 mi. N and 20 mi. W Ocampo, 4150 ft., 1; 18 mi. S and 14 mi. E Tanque Alvarez, 4000 ft., 4; 1-1/2 mi. NW Ocampo, 3300 ft., 6; _Ocampo_, 1; 5 mi. N and 19 mi. W Cuatro Ciénegas, 3250 ft., 1. +Cratogeomys castanops consitus+ Nelson and Goldman 1934. _Cratogeomys castanops consitus_ Nelson and Goldman, Proc. Biol. Soc. Washington, 47:140, June 13, type from Gallego, 5500 ft., Chihuahua. _Distribution._--Arid high plains from central Chihuahua, east and southeast at least into northwestern Coahuila (see fig. 1). _Comparisons._--From _Cratogeomys castanops lacrimalis_ Nelson and Goldman, specimens from Boquillas and Marathon north of the Río Grande in Texas, _consitus_ differs in: Body smaller; tail and hind foot shorter; upper parts paler, more light buffy and less rufous; underparts paler, light buffy instead of dark buffy; skull decidedly smaller; zygomata slightly less widely flaring; palate especially shorter; rostrum narrower; squamosal breadth less; incisors smaller. From topotypes of _Cratogeomys castanops clarkii_, found to the north along the Río Grande, _consitus_ differs in: Body smaller; tail and hind foot shorter; upper parts paler, more buffy and less rufous; skull markedly smaller, especially in palate and nasals; zygomata less widely flaring; tympanic bullae more inflated; mastoid bullae less inflated; basioccipital parallel-sided as opposed to wedge-shaped. From _Cratogeomys castanops convexus_, found to the east, _consitus_ differs in: Body smaller; tail and hind foot shorter; upper parts paler, more buffy and less ochraceous; underparts paler, white or light buffy instead of pale ochraceous; skull smaller; zygomata less widely flaring; palate shorter; rostrum decidedly narrower and, relative to length of skull, longer; squamosal breadth less; incisors smaller. From _Cratogeomys castanops excelsus_, found to the south, _consitus_ differs in: Size smaller; tail and hind foot shorter; upper parts darker, more rufous and less pinkish-buff; skull conspicuously smaller, especially in palate, rostrum, and nasals; zygomata less