_Type._--Female, adult, skull and skin, No. 27234, Mus. Nat. Hist., Univ. Kansas, from Mustang Island, 14 mi. SW Port Aransas, Aransas County, Texas; obtained 30 June 1948 by W.K. Clark; original No. 543. _Range._--Known from Mustang Island only. _Diagnosis._--Size medium (see measurements). Color pale, and as described for _D. o. parvabullatus_. Skull small; auditory bullae (actually and relative to remainder of skull) smaller than in any other known kind of _Dipodomys_, except _D. o. parvabullatus_ in which breadth across bullae is approximately the same; notably narrow across maxillary processes of zygomatic arches. _Comparisons._--From _Dipodomys ordii sennetti_ (J.A. Allen) of the mainland, _D. o. largus_ differs in: Color paler on pigmented areas; white areas more extensive; skull averaging smaller except in basilar length and length of nasals which are approximately the same as in _D. o. sennetti_. From _Dipodomys ordii compactus_ True of Padre Island, _D. o. largus_ differs in: Body longer; tail shorter; skull narrower across tympanic bullae and across maxillary processes of zygomatic arches; nasals shorter. From _Dipodomys ordii parvabullatus_ of the coastal island south of Padre Island, along the gulf coast of Tamaulipas, _D. o. largus_ differs in: Body and tail longer; basilar length of skull averaging less; breadth across maxillary processes of zygomatic arches greater; premaxillae not extending so far behind nasals. _Remarks._--_D. o. largus_ resembles _D. o. compactus_ in external proportions and _D. o. parvabullatus_ in cranial proportions. The degree of difference between _D. o. compactus_ and _D. o. largus_ is less than between _D. o. compactus_ and _D. o. parvabullatus_. To me, the three subspecies mentioned in the preceding sentence are indistinguishable in color. Two of the eleven specimens of _D. o. largus_ are of the light color phase (upper parts Cartridge Buff) whereas all but two of the eleven specimens of _D. o. compactus_ are of the light color phase. Each of the cheek-teeth of the upper jaw of _D. o. largus_ has a complete ring of enamel around the dentine of the occlusal surface, as described by Setzer (Univ. Kansas Publ., Mus. Nat. Hist., 1:517, December 27, 1949) for _D. o. compactus_. The upper dentitions of ten specimens of _D. o. compactus_ examined by me in this respect reveal a total of only five teeth (in four individual animals) that have the enamel ring incomplete; one premolar and three molars are incomplete on the lingual side and one molar is incomplete on the labial side.