We are indebted to Dr. Richard H. Manville for arranging a loan of specimens of _Geomys personatus tropicalis_ in the United States National Museum. Dr. Marshall Johnston kindly identified specimens of plants from the barrier island. Several bones of birds and mammals were identified by Dr. Pierce Brodkorb and Dr. E. L. Lundelius. Mr. J. Knox Jones identified some of the mammalian material, and Dr. W. E. Duellman verified the identifications of the lizards; we thank all of these men for their willing assistance. The Ecological Setting The barrier island of Tamaulipas geologically and ecologically resembles Padre Island, of the Gulf coast of lower Texas, north of the mouth and delta of the Rio Grande. South of the delta, the island in Tamaulipas is a narrow strip of sand less than a mile in average width and is broken by a series of narrow inlets or "passes" through which water from the Gulf of Mexico mingles with that of the Laguna Madre de Tamaulipas. The passes are subject to recurrent opening and closing. North of the mouth of the Río Soto la Marina, eight passes are designated by local fishermen, but only three, the Third, Fourth, and Fifth, were open at the time of our visit. The Laguna Madre de Tamaulipas is described by Hildebrand (1958) in connection with a preliminary study of the fishes and invertebrates there. The average depth is probably less than 70 cm. and the waters are hypersaline. In the time of the recent drought in Texas and northeastern México, salinity varied from 108 to 117 parts per thousand in the northern part of the laguna near Arroyo del Tigre (measurements taken in March, 1955) to from 39 to 48 parts per thousand in the southern part near Punta Piedras (measurements taken in October and November, 1953, and in March, 1954). Discussions of the geologic history, ecology, and zoogeography of the lagoons of the Gulf coast of the United States are given by Hedgpeth (1947; 1953). Localities in coastal Tamaulipas mentioned in the text of this paper are shown on Plate 5. The principal animal habitats are found in three vegetational associations (plates 6 and 7). On flats and low dunes lying between, and partly sheltered by, larger active dunes, small clumps of _Croton punctatus_ and a sedge (_Fimbristylis castanea_) are the only conspicuous plants. Near the western edge of the dunes, _Ipomoea pescaprae_ var. _emarginata_ is mixed with _Croton_, and there are scattered clumps of shrubby wolf-berry (_Lycium carolinianum_ var. _quadrifidum_), and mesquite (_Prosopis juliflora_).