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As is so often the case in taxonomy, an attempt to understand the species collected for other work leads to the need for a revision of a genus in one’s area. The difficulty of dealing with the North American material of the very common genus Plysica has led to the present attempt to understand its species and their variations. In this study the author has been much helped and influenced by the recent European papers on the genus by Lynge (1935, 1916); Schade (1938); DuRietz (1925); Nadvornik (1947, 1948); and MaasGeesteranus (1952). The North American papers of Degelius (1940, 1941) also aided in interpretations. The bulk of the information in this paper is based on the examination of over 10,000 packets of Physica in the major herbaria of North America.