The neotropical genus Potamium as described by Mitten (1869) included species that had little more in common than a growth form adapted to a semi-aquatic habitat. Brotherus divided the genus in section Eu-Potamium and section Potamiopsis, based on peristome characters. This study shows that all species of section Eu-Potamium belong to Sematophyllum, including P. lonchophyllum which was designated as lectotype of Potamium by Buck (1990). In section Potamiopsis two species are recognized: P. vulpinum (Mont.) Mitt. (syn. nov.: Sematophyllum maguireorum Buck) and P. deceptivum Mitt. (syn. nov.: P. leucodontaceum (C.Müll.) Broth., Meiothecium negrense Mitt. and Maguireella vulpina (Mont.) Buck). The genus Potamium is characterized by capsules with an endostome consisting of a low basal membrane and filiform segments, which are often reduced in P. deceptivum. P. vulpinum is designated as lectotype of Potamium and the lectotypification of Buck is rejected. Descriptions, illustrations and a key are provided to the species of Potamium and allied semiaquatic taxa of Sematophyllum
