_Range._--Nuevo León, extreme northern Zacatecas, Tamaulipas and northern San Luis Potosí, Mexico; marginal occurrences are: Coahuila, La Ventura; Tamaulipas, Tula, Juamave and Nuevo Laredo. _Diagnosis._--Size medium (see measurements). Color dark, entire dorsal surface (_a_) between Cinnamon-Buff and Clay Color, purest on sides, flanks and cheeks, upper parts strongly suffused with black; posterior surfaces and fold of pinnae of ears, white; arietiform markings, plantar surfaces of hind feet, inside of pinnae of ears, dorsal and ventral stripes of tail, blackish. Skull of medium size; nasals relatively short; interorbital width narrow; auditory bullae relatively as well as actually greatly inflated posteriorly, laterally and ventrally; braincase vaulted; zygomatic arches light and straight; pterygoid fossae ovoid; rostrum short and wide. _Comparisons._--From _Dipodomys ordii palmeri_, _D. o. fuscus_ differs in: Size larger; color lighter; skull larger; rostrum shorter and wider; interorbital width less; nasals shorter; auditory bullae more expanded laterally, posteriorly and ventrally; interparietal region narrower; pterygoid fossae more circular; upper incisors wider at cutting edge; zygomatic arches straight rather than bowed laterally. From _Dipodomys ordii obscurus_, _D. o. fuscus_ differs as follows: Size larger; color lighter; skull larger; rostrum wider and shorter; interorbital region wider; width across maxillary arches greater; auditory bullae more expanded laterally, posteriorly and ventrally; zygomatic arches light and straight instead of heavy and bowed laterally; pterygoid fossae more nearly circular; braincase more vaulted; foramen magnum smaller. From _Dipodomys ordii ordii_, _D. o. fuscus_ differs in: Size larger; color darker; skull larger; rostrum shorter and wider; nasals short and more flared distally; interorbital width greater; auditory bullae more inflated posteriorly, laterally and ventrally; interparietal region narrower; width across maxillary arches greater; pterygoid fossae larger and more nearly circular; braincase more vaulted. From _Dipodomys ordii sennetti_, _D. o. fuscus_ can be distinguished by larger size, longer tail, darker color, and cranially by larger auditory bullae, longer nasals, wider interorbital region and greater width across maxillary arches. _Remarks._--This dark subspecies of _Dipodomys ordii_ is most closely allied to _Dipodomys ordii palmeri_ from, which it can be distinguished by the characters set forth under comparisons.