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).

The dunes are relatively stabilized on the western side of the island, and there we found moderately dense stands of mesquite trees reaching heights of from eight to 10 feet. Prickly-pear cactus (Opuntia lindheimeri) was common in those stands of mesquite, and we saw an occasional yucca tree. A fairly dense ground cover was formed by blanket-flower (Gaillardia pulchella), marsh-elder (Iva sp.), Flaveria oppositifolia, Enstoma exaltatum, and Croton capitatus var. albinoides.

A more open, xeric expression of the mesquite-cactus vegetation occurs on exposed, low clay dunes (see description by Price, 1933) located on alkaline flats bordering the laguna. At the time of our visit, most of the mesquites in these stands were dead or dying, the cactus was abundant, and the ground cover, which was sparse, included drop-seed (Sporobolus virginicus), ragweed (Ambrosia psilostachya), and Commicarpus scandens.

On alkaline flats flooded by hypersaline waters of the laguna following heavy rains, Batis maritima is found in the lower areas, but on the slightly elevated areas there is low and almost continuous cover of MonanthochloË littoralis, in which can be found Batis, Borrichia fructescens, Salicornia sp., Iva sp., and sea-lavender (Limonium carolinianum).

Near Third Pass, sea oats (Uniola paniculata), evening primrose (Oenothera sp.), and cordgrass (Spartina sp.) are present on the dunes, and on alkaline flats we collected Conocarpus erectus, Leucaena sp., and Cassia fasciculata var. ferrisiae.