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Abstract
Fragilaria mesolepta Rabenh. was first described by Ludwig Gottlob Rabenhorst on the label to no. 1041 of his exsiccata, Die Algen Europas. The label provides collection information in addition to a description of the species and is accompanied by a small sheet with two figures. Among the various sets of the exsiccata that were distributed, the specimens in packets labeled 1041 were found to include diatoms with three different morphologies. Only specimens of one of these morphologies conform to the illustration and description given by Rabenhorst. We designate as lectotype of F. mesolepta an individual on slide 3616 in the Farlow Herbarium.
We also examined the type material of Fragilaria subconstricta Østrup and Fragilaria tenuistriata Østrup, which were considered synonyms of F. mesolepta by Lange-Bertalot in Krammer & Lange-Bertalot (1991). Both taxa are not synonyms of F. mesolepta, but different species and can be identified using shape and position of a rimoportula.
