We obtained specimens of the spotted ground squirrel (_Citellus spilosoma_), Ord kangaroo rat (_Dipodomys ordii_), hispid cotton rat (_Sigmodon hispidus_) and black-tailed jack rabbit (_Lepus californicus_). Tracks and other sign of the coyote (_Canis latrans_) were seen. So far as we could ascertain, by our own investigations and from our Mexican hosts at the fishing camp, no other kinds of native mammals lived on the island. The ground squirrel and kangaroo rat were found by us on only the sandy areas where there were xerophitic shrubs. The cotton rat was found only in the grass. The jack rabbit and coyote ranged over the whole of the island excepting the areas of plum brush in which we saw no sign of any mammal. To answer the second of our initial questions: The affinities of the mammals of the barrier beach of Tamaulipas are approximately equally divided between those of the mainland and those of Padre Island. The ground squirrel is indistinguishable from the subspecies which occurs both on the mainland and Padre Island to the northward; the other three kinds of mammals of which we obtained specimens prove to be subspecifically distinct from any previously named kinds and seem to be confined to the off-shore beach. Accounts of these four mammals and of a previously unnamed subspecies of kangaroo rat on Mustang Island, Texas, follow. Citellus spilosoma annectens (Merriam) Spotted Ground Squirrel 1893. _Spermophilus spilosoma annectens_ Merriam, Proc. Biol. Soc. Washington, 8:132, December 28, type from "The Tanks," 12 mi. from Point Isabel, Padre Island, Texas. 1904. [_Citellus spilosoma_] _annectens_, Trouessart, Catalogus Mammalium ..., p. 340. Thirteen specimens (Nos. 35441-35453) were collected. All are from the north side of the tidal inlet. Although the ground squirrels were easily trapped, it was difficult to obtain a perfect skin because the gulls (_Larus_ sp.) pulled the skin off of the distal part of the tail as soon as a squirrel was secured in a trap. The specimens seem not to differ from Texan specimens from the type locality and Mustang Island. Dipodomys ordii parvabullatus new subspecies Ord Kangaroo Rat _Type._--Male, adult, skull and skin, No. 35454, Mus. Nat. Hist. Univ. Kansas, from island, 88 miles south and 10 miles west of Matamoros, Tamaulipas, Mexico; obtained 19 March 1950 by E.R. Hall and Curt von Wedel; original No. 6778 E.R. Hall. _Range._--Islands along coast of Tamaulipas, Mexico.