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Abstract
The diatom genus Libellus Cleve was erected for naviculoid species with numerous girdle bands, L. grevillii (CA. Ag.) Cleve being the type species of the genus. The structure of this species has been compared with that of Navicula tripunctata (Müll.) Bory, the type of Navicula Bory. There are considerable structural differences between these two, on the basis of which N. delognei Van Heurck (L. grevillii) cannot remain in a genus typified by N. tripunctata. For nomenclatural reasons Libellus Cleve cannot simply be re-erected to accommodate N. delognei and other species of similar structure, and so the new genus Parlibellus is established for them. The genus is described, as is a new term for a structure at present peculiar to Parlibellus.
Members of Navicula sect. Microstigmaticae sensu Hustedt were also investigated. Some are placed in Parlibellus, others are excluded. Species previously included in Libellus have been examined and their taxonomie position considered. The species now placed in Parlibellus are described and the necessary nomenclatural changes made. Reasons for the exclusion of other species are given, and where possible, their taxonomie position is indicated.