Type.—Male, no. 490/1631, U. S. Nat. Mus.; near Fort Massachusetts, Costilla County, Colorado; obtained in 1853 by F. Kreutzfeldt (Pacific Railroad Survey).
Range.—The San Luis Valley of south-central Colorado and north-central New Mexico; marginal occurrence are: in Colorado, 22 mi. E Mosca, Saguache, Alamosa and Antonito; in New Mexico, 4 mi. SW Cimmaron.
Diagnosis.—Size small (see measurements). Color dark, entire dorsal surface between (c) Vinaceous-Buff and Avellaneous, purest on sides and flanks, upper parts strongly suffused with black; arietiform markings, pinnae of ears, plantar surfaces of hind feet and dorsal and ventral stripes of tail, blackish. Skull small; rostrum slender; interorbital width narrow; auditory bullae strongly inflated; braincase but slightly vaulted; interparietal region wide; zygomatic arch bowed laterally.
Comparisons.—From Dipodomys ordii richardsoni, D. o. montanus differs as follows: Size smaller; color darker in all pigmented areas; ventral stripe of tail continuous to end of pencil in all specimens, whereas in D. o. richardsoni the stripe is incomplete in most specimens. In all specimens the dorsal and ventral stripes of the tail are darker in D. o. montanus; skull smaller in all measurements taken; jugal straight or nearly so, as contrasted to bowed laterally; pterygoid fossae shallower and more ovoid, that is to say, wings of pterygoid flare laterally; ventral projection of auditory bullae more pronounced; rostrum, immediately in front of zygomatic process of maxilla, nearly parallel to median plane along long axis of skull, while in D. o. richardsoni this union forms a decided obtuse angle; paroccipital processes smaller; foramen magnum smaller and more ovoid as opposed to larger and more circular.
From Dipodomys ordii evexus, D. o. montanus differs as follows: Size smaller in all measurements taken except width across auditory bullae which is greater; color darker, ventral stripe of tail continuous to end of pencil; least width of supraoccipital less (2.5 mm. in D. o. montanus and 3.5 mm. in D. o. evexus); extension of premaxillae posterior to nasals greater; braincase more inflated dorsally; bullae relatively as well as actually larger and more inflated ventrally; pterygoid fossae narrower anteroposteriorly and wider laterally than in D. o. evexus; jugal straight or nearly so as contrasted to bowed laterally; lacrimal processes smaller.
For comparison with Dipodomys ordii medius see account of that subspecies.
Fig. 26. Known occurrences and probable geographic range of the subspecies of Dipodomys ordii in the southwestern fourth of the range of the species. 14. D. o. ordii 17. D. o. chapmani 31. D. o. longipes