The geographic and stratigraphic occurrence of the Middle Triassic sauropterygian genera Ceresiosaurus, Lariosaurus, and Silvestrosaurus is summarized, and their skeletal morphology is critically reviewed. A cladistic analysis using parsimony supports the synonymy of all three genera, among which Lariosaurus takes priority. The genus Lariosaurus includes five species, balsami, buzzii, calcanii, curionii n. sp., and valceresii, with only partially resolved interrelationships. The sister group of Lariosaurus is the genus Nothosaurus. Lariosaurs diversified during Ladinian times in the northwestern Tethyan coastal area, after the genus Nothosaurus had invaded the Alpine intraplatform basin facies Includes bibliographical references (p. 40-43) Fieldiana series has been published as Geological Series by Field Columbian Museum (1895-1909) and Field Museum of Natural History (1909-1943), and as Fieldiana: Geology by Chicago Natural History Museum (1945-1966) and Field Museum of Natural History (1966-1978). Fieldiana Geology New Series No. 1 began June 29, 1979