The Revillagigedo Archipelago consists of a group of four volcanic, oceanic islands located approximately 210 miles southwest of the tip of Baja California. They are located between 180 20' and 190 20' N. latitude and 1100 45' and 1140 50' W. longitude and are governed by the Mexican state of Colima. The islands, in order of increasing size, are Roca Partida, San Benedicto, Clarion, and Socorro. They are all uninhabited and there is no fresh water on any of the islands except a small stagnant well and some rock seeps on Socorro. The rainy season, according to Hanna (1926), probably occurs from September to December. The only resident vertebrates are birds, introduced sheep (on Socorro Island where they were left in 1867 and are present today only in small numbers), and three species of reptiles. No amphibians or freshwater fishes occur on the islands. No reptiles occur on Roca Partida or San Benedicto islands. The former is a small barren rock and the latter is a steep sided island four miles long and about 2 miles wide. It is on San Benedicto that a new volcano, Boqueron, first erupted in August of 1952 (Williams, 1952; Richards, in press). Clarion Island (6 by 4 miles) is covered with grass and thick clumps of Convolvulus-covered prickly pear cactus plus large areas of low shrubs, 5-6 feet high. Clarion island reaches, an elevation of 1,000 feet. Its herpetofauna consists of a racer, Masticophis anthonyi, a lizard, Urosaurus clarionensis, and two sea turtles, Caretta c. gigas and Chelonia mydas agassizi. Socorro Island (9 by 9 miles) reaches an elevation of 3,710 feet. The dominant vegetation of the west, south and east sides of the island consists of one species of low shrub and a prickly pear cactus. The north side of the island and the elevations above 2,000 feet are covered with broad-leaved tropical plants. The herpetofauna of Socorro consists of one species of lizard, Urosaurus auriculatus and one sea turtle, Chelonia mydas agassizi. The islands have been studied biologically only during short stops by major scientific expeditions. Most important of these were the Albatross Expedition (Townsend, 1890) and the California Academy of Sciences Expedition of 1925 (Hanna, 1926; Slevin, 1926). Other expeditions to the islands, as well as the complete history of the islands, can be found in Mares y Islas Mexicanos del Pacifico, Mexico, 1949. The reader is referred to Hanna (1926), Johnston (1931), McLellan (1926), Richards (MS), and Brattstrom and Howell (MS) for further descriptions of the islands, their plants, birds, and geology. In March of 1953, through the generosity of Scripps Institution of Oceanography, I had the opportunity of visiting the Revillagigedo Archipelago during a cruise undertaken primarily to study the geology and volcanology of the islands. The trip was led by Mr. Adrian Richards aboard the Research Ship Paolina-T, Robert Newbegin, Captain. The ship left San Diego on March 3
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