_Diagnosis._--Previously known from only one specimen, a subadult female, this subspecies has not been well diagnosed. At hand we have five near topotypes of _convexus_ (including two adult females and one adult male) and specimens assignable to this subspecies from several other localities. This subspecies may be characterized as follows: Size medium (see tables 1 and 2); dorsal profile of skull convex in females and flat, especially posteriorly, in males; zygomata weakly constructed and not widely flaring; mastoid and tympanic bullae inflated; nasals short; rostrum broad and short; maxillary teeth large. [Illustration: ~Fig. 1.~ Geographic ranges of the subspecies of _Cratogeomys castanops_ found in Coahuila, México. Guide to subspecies 4. _C. c. bullatus_ 8. _C. c. subsimus_ 1. _C. c. convexus_ 5. _C. c. ustulatus_ 9. _C. c. goldmani_ 2. _C. c. consitus_ 6. _C. c. jucundus_ 10. _C. c. subnubilus_ 3. _C. c. sordidulus_ 7. _C. c. excelsus_ 11. _C. c. planifrons_] _Comparisons._--From topotypes of _Cratogeomys castanops angusticeps_ Nelson and Goldman, found to the north and east across the Río Grande in Texas, _convexus_ differs in: Body larger; upper parts more reddish, especially on sides; skull with zygomata less heavy, nasals broader, pterygoids smaller, maxillary teeth larger. For comparisons of _convexus_ with the subspecies of _C. castanops_ found to the west, south and southeast, see accounts of the subspecies to follow. _Remarks._--The geographic range of _convexus_ is restricted, being bounded on the west and southwest by mountains, especially the Serranías del Burro, and on the north and east by the Río Grande. The range of the subspecies found to the southeast may not be continuous with that of _convexus_. At least, in the area between Villa Acuña and Piedras Negras, along the Río Grande, no specimens were obtained and no sign was observed. We suspect that in this area the species occurs only locally if at all. A specimen taken near the Río Grande in Coahuila, opposite Samuels, Texas, and assigned to _Cratogeomys castanops clarkii_ by Nelson and Goldman (op. cit.:140), has been examined by us and is referable to _convexus_. This specimen is typical of _convexus_ except for the lesser inflation of the mastoid bullae and tympanic bullae. Conspicuous differences between _convexus_ and _angusticeps_ indicate that the Río Grande is an effective barrier to passage by these rodents. _Specimens examined._--Total, 14, all from Coahuila: Río Grande, 17 mi. S Dryden, Terrell Co., Texas, 6; Río Grande, opposite Samuels, Val Verde Co., Texas, 1 (BSC); Villa Acuña, 5; Cañon del Cochino, 21 mi. E and 16 mi. N Piedra Blanca, 1; 11 mi. W Hda. San Miguel, 1. +Cratogeomys castanops bullatus+ new subspecies _Type._--Female, adult, skin and skull, No. 48498, Univ. Kansas Mus. Nat. Hist., 2 mi. S and 6-1/2 mi. E Nava, 810 ft., Coahuila; 16 June 1952; obtained by Robert J. Russell, original number 276.