A Revision of the Genus Potamium (Musci: Semato- phyllaceae)

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. Potamium is a small genus in the Sematophyllaceae with a distribution confined to the Amazon basin and the Guianas. Since its foundation by Mitten in 1869, it has never been a clear-cut genus. Mitten included 7 species which had the following characters in common: inconspicuous alar cells, an erect or little-inclined capsule on a short seta, and growing in a periodically submerged habitat. Brotherus (1908),transferred 3 of these species to other genera, i.e. P. pulchellum (Hook.) Mitt. and P. casiquiariense Spruce ex Mitt. to Pterogonidium and P. commutatum (C.Müll.) Mitt. to Meiotheciopsis (= Donellia Buck 1988). The remaining species were P. vulpinum (Mont.) Mitt. and P. deceptivum Mitt. with creeping stems and a peristome with filiform endostome segments, and P. lonchophyllum (Mont.) Mitt. and P. pacimoniense Spruce ex Mitt. with floating stems and a peristome with broad, keeled endostome segments. Based on these characters Brotherus divided the genus in two sections, section EuPotamium and section Potamiopsis. He added 4 other species (Table 1).