Re-examination of the type material of Frustulia ventricosa Agardh and F-operculata Agardh (Rhopalodia operculata (Agardh) Hakansson)

No lectotype of either Frustulia ventricosa Agardh or F. operculata Agardh has been designated until now. Agardh's type material of these two species was therefore re-examined. There are a number of species of diatom present in this material, but only a Rhopalodia and a Nitzschia are abundant. The s pecimens that best correspond to Agardh's protologue of Frustulia ventricos a are large examples of the Rhopalodia and those that best correspond to th e protologue of F. operculata are small examples of the same species. Indiv idual specimens of this species are designated as the lectotypes of these t wo specific names. Its correct name is Rhopalodia operculata (Agardh) Hakan sson (1979). The name Cymbella ventricosa Agardh, which has the same type a s Frustulia ventricosa Agardh, was misapplied to Encyonema minutum (Hilse) D.G. Mann until 1975, and Frustulia operculata Agardh, the type species of both Pyxidicula Ehrenberg and Cyclotella (Kutzing) Brebisson, was misapplie d to Cyclotella distinguenda Hustedt until 1984. Cyclotella and Rhopalodia have both been conserved, preserving long established usage, and no further conservation is necessary to preserve current usage.