Type.—Male, no. 93477, Mus. Vert. Zool., Univ. California; Yellowstone River, 5 mi. W Forsyth, 2,750 ft., Rosebud County, Montana; obtained on June 2, 1940, by J. R. Alcorn, original no. 1528.
Range.—Extreme southwestern Saskatchewan and southeastern Alberta, eastern half of Montana, northern Wyoming and probably extreme western North Dakota; marginal localities are: 50 mi. W Swift Current, Saskatchewan; "near Medicine Hat," Alberta; in Wyoming, Sheep Creek, and 23 mi. SW Newcastle; in Montana, Medicine Rocks (14 mi. NE Ekalaka), and Glendive.
Diagnosis.—Size large (see measurements). Color dark, entire dorsal surface near (c) Ochraceous-Buff, purest on sides and flanks; upper parts mixed with black; arietiform markings, pinnae of ears, dorsal and ventral stripes of tail and plantar surfaces of hind feet brownish-black. Skull large; rostrum short, wide and deep; braincase slightly vaulted; auditory bullae markedly inflated ventrally; zygomatic arches heavy and bowed laterad; upper incisors long and robust.
Comparisons.—From Dipodomys ordii priscus, D. o. terrosus differs as follows: Size larger in all measurements taken, except for length of hind feet, which is less; color darker in all pigmented areas; skull larger in all parts measured except width of interparietal, which is less; auditory bullae more inflated ventrally; zygomatic processes of maxillae wider; rostrum deeper and shorter.
From Dipodomys ordii richardsoni, D. o. terrosus differs as follows: Size larger; color darker in all pigmented areas; ventral stripe of tail extending farther distally; skull larger except in width across auditory bullae, which is the same.
For comparison with Dipodomys ordii luteolus see account of that subspecies.
Remarks.—As noted in the comparisons, D. o. terrosus is larger and darker than D. o. priscus, D. o. luteolus or D. o. richardsoni, its closest geographic neighbors, and does not resemble any of them, but rather resembles D. o. longipes and D. o. evexus in size and appearance, both of which are distantly removed geographically.
Like other subspecies of the species D. ordii, D. o. terrosus prefers sandy soils to those of any other type. Two miles east and 1 mile south of Forsyth, Montana, animals were trapped on lenses of sandy soil. These lenses alternated with areas of black loam of similar size. It was noteworthy that burrows were found only in the areas of sandy soil, although paths used by the rats when foraging did extend onto and several crossed the lenses of black loam. We were not permitted to excavate any of these burrows, but conversation with farmers of the immediate vicinity indicated that the burrows were not deep. An eight-inch disc would frequently plow out nests and food caches. It was said that each of several caches contained as much as a peck of wheat.
Intergradation was noted in animals from 23 miles southwest of Newcastle and Arvada, Wyoming. In animals from both localities the pterygoid fossae are more as in D. o. luteolus but referable to D. o. terrosus. The specimens from Arvada, although immature, possessed cranial characters which were intermediate between those of D. o. terrosus and D. o. luteolus but the specimens are referable to the former.
Specimens examined.—Total, 74, distributed as follows: