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.
It seems that the Rio Grande serves as a barrier to the dispersal southward of kangaroo rats from the north side and vice versa. D. o. fuscus ranges from the type locality to Nuevo Laredo and has not been found on the north side of the river, being replaced there, up-river, by Dipodomys ordii medius and down-river by Dipodomys ordii sennetti.
Specimens examined.—Total, 53, distributed as follows:
Coahuila: San Juan Neponuceno, 5 mi. N La Ventura, 4 (MVZ); La Ventura, 7 (USBS).
Tamaulipas: Nuevo Laredo, 2 (USBS); Miquihuana, 14 (4 USBS; 9 MVZ; 1 AMNH); Juamave, 8 (USBS); Tula, 5 (UM); 3 mi. N Lulú, 5 (MVZ): Lulú, 8 (MVZ).