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Abstract
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 specimens that best correspond to Agardh's protologue of Frustulia ventricosa are large examples of the Rhopalodia and those that best correspond to the protologue of F. operculata are small examples of the same species. Individual specimens of mis species are designated as the lectotypes of these two specific names. Its correct name is Rhopalodia operculata (Agardh) Håkansson (1979). The name Cymbella ventricosa Agardh, which has the same type as 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 (Kätzing) Brebisson, was misapplied 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.