_Perodipus montanus richardsoni_, Bailey, N. Amer. Fauna, 25:144, October, 1905 (part--the part from Santa Rosa, New Mexico). _Type._--Male, no. 118526, U. S. Nat. Mus. Biol. Surv. Coll.; Santa Rosa, Guadalupe County, New Mexico; obtained on October 5, 1902, by Jas. H. Grant, original no. 565. _Range._--From north-central New Mexico, southeastward to west-central Texas; marginal localities are, in New Mexico: 15 mi. N Ojo Caliente, Gallina Mts., Deer Creek, San Pedro; in Texas: 20 mi. N Monahans, Colorado, 7 mi. E Post, 6 mi. SW Muleshoe. _Diagnosis._--Size medium (see measurements). Color dark; entire dorsal surface (_14''_) between Orange-Cinnamon and Cinnamon, purest on sides and flanks, dorsal surface lightly washed with black; arietiform markings, pinnae of ears, plantar surfaces of hind feet, dorsal and ventral stripes of tail, brownish-black. Skull of medium size; nasals long; medial mastoid portion of auditory bullae well inflated caudad; braincase vaulted; external auditory meatus small; rostrum short and truncate; medial auditory portion of auditory bullae relatively little inflated; pterygoid fossae ovate; zygomatic arches slender and relatively straight; junction of jugal and zygomatic process of maxilla heavy. _Comparisons._--From _Dipodomys ordii richardsoni_, _D. o. medius_ differs as follows: Tail longer; hind foot shorter; color darker; arietiform markings more distinct; white lateral stripes of tail narrower; ventral stripe of tail in most specimens complete to end of pencil; postauricular spots less pronounced; hip stripe narrower and in some specimens almost obliterated; skull smaller in all measurements taken; angle of dorsal extension of premaxilla with zygomatic process of maxilla more nearly 90°; braincase more vaulted; medial mastoid portion of auditory bullae more inflated, and coming to more of a point; medial auditory portion of auditory bullae more inflated ventrally; rostrum shorter and narrower; external auditory meatus smaller. From _Dipodomys ordii montanus_, _D. o. medius_ differs in: Color lighter in all pigmented areas; skull larger in all respects; rostrum shorter and heavier; bullae more inflated; zygomata, while nearly straight, are bowed slightly laterally; pterygoid fossae more ovate; foramen magnum larger; pterygoid foramina smaller. Compared with _Dipodomys ordii ordii_ and _Dipodomys ordii sennetti_, _D. o. medius_ is larger and darker. The skull is also larger in all measurements taken. Compared with _Dipodomys ordii longipes_, _D. o. medius_ is darker and smaller. _Remarks._--This hitherto undescribed race of _Dipodomys ordii_ can readily be distinguished from any of its near neighbors by the characters set forth under diagnosis and comparisons.