From Dipodomys ordii ordii, D. o. oklahomae differs in: Size larger in all measurements taken; color lighter in all pigmented areas; ventral stripe of tail extending one fourth length of tail rather than to end; skull larger in all respects; rostrum heavier; zygomatic arches heavier; bullae more inflated ventrally; cutting edge of upper incisors wider; pterygoid fossae larger; braincase more vaulted; nasals more expanded distally; orbital region larger; interparietal region wider.
Remarks.—Trowbridge and Whitaker named this kangaroo rat as a full species. The diagnostic characters were the length and breadth of the rostrum and the relatively great inflation of the auditory bullae. Also, Dipodomys oklahomae was not known to intergrade with any other named kinds. [Davis] (1942:332) treated D. oklahomae as a subspecies of the earlier named species Dipodomys ordii. Certain characters in specimens from the type series of both D. o. richardsoni and D. oklahomae, such as the shape and configuration of the nasals, the over-all proportion of the skull, tooth pattern and body proportions through individual variation overlap and indicate that these two groups of animals belong to the same species, even though animals from intermediate geographic areas are not available to show actual intergradation. My findings corroborate [Davis'] conclusion that D. oklahomae should stand as Dipodomys ordii oklahomae. In spite of the great similarities shown by the two groups of animals there are still sufficient diagnostic characters between the two groups to enable them to be segregated easily as valid subspecies.
Dipodomys ordii oklahomae is, for some unknown reason, restricted to a limited geographic range. Specimens examined from the upper reaches of the South Canadian River, farther westward, are all referable to D. o. richardsoni rather than, as would be expected, to D. o. oklahomae since the habitat for these animals is continuous from the type locality of D. o. oklahomae to the upper reaches of the South Canadian River. In length and shape of the nasals, degree of inflation of the auditory bullae and width of the interorbital region these specimens from the upper reaches of the South Canadian River are intergrades between D. o. richardsoni and D. o. medius. The range of D. o. medius lies to the south of that of D. o. richardsoni and to the southwest of that of D. o. oklahomae.
The present range of D. o. oklahomae, as now understood, is the most eastern part of the range of the species Dipodomys ordii and of the genus Dipodomys. The existence of D. o. oklahomae in this area is a precarious one since its habitat is limited in extent and is periodically flooded.
Although no specimens are known from the area where intergradation between D. o. oklahomae and D. o. richardsoni would be expected to occur, it would seem that when animals from this region become available, intergradation will be shown to occur.
Specimens examined.—Total, 8, all from Oklahoma, distributed as follows: Grady County: 4 mi. N Minco, 1 (USBS). Cleveland County: 2-1/4 mi. S Norman, 7 (6 OU; 1 USBS).
Dipodomys ordii compactus True
Dipodomys compactus True, Proc. U. S. Nat. Mus., 11:160, January 5, 1889.
Cricetodipus compactus, Trouessart, Catalogus Mammalium, 1:581, 1897.
Perodipus compactus, Elliot, Field Columbian Museum, Zool. Ser., 2:240, 1901.