Two specimens from Bagdad, Tamaulipas, in the delta of the Rio Grande (Nos. 116485 and 11487, U.S.N.M., Biol. Surv. Coll.), are referred to _D. o. compactus_ on basis of long body and long tail. The specimens, both Light Ochraceous-Buff, are so young that not all of the enamel is worn off the crowns of the cheek-teeth. Specimens of _D. o. compactus_, _D. o. parvabullatus_ and _D. o. sennetti_ of comparable age are not available, and it, therefore, is impossible to know whether size and shape of the skull in the population at Bagdad are the same as they are in _D. o. compactus_ of Padre Island. _Specimens examined._--Total, 11, all from Texas. Aransas County: Mustang Island, 14 mi. SW Port Aransas. Sigmodon hispidus solus new subspecies Hispid Cotton Rat _Type._--Male, adult, skull and skin; No. 35468, Mus. Nat. Hist., Univ. Kansas; from island, 88 mi. S and 10 mi. W Matamoros, Tamaulipas, Mexico; 22 March 1950; obtained by E.R. Hall and Curt von Wedel; original No. 6806 E.R. Hall. _Range._--Known from the type locality only but probably occurring on most of the chain of islands off the coast of Tamaulipas. _Diagnosis._--Small; hind foot short; rostrum broad. _Comparison._--From its nearest relative, geographically and morphologically, _Sigmodon hispidus berlandieri_ Baird of the adjacent mainland, _S. h. solus_ differs in smaller size, and a rostrum that is broader in relation to the length of the skull. _Remarks._--On the last night of our stay on the island, traps set in grass approximately 20 inches high, yielded one pair of _Sigmodon_. The color is lighter than in the average of specimens from the mainland (for instance those from Victoria and Soto la Marina) but can be matched by selected specimens. In animals of equal age, the hind foot and basilar length are shorter in _S. h. solus_ than in _berlandieri_. The broadening of the rostrum, which occurs with advanced age, is attained in _solus_ when the skull is yet short; the maximum breadth of the rostrum in the adults is more, instead of less, than a fourth of the basilar length.