Fragilaria pinnata Ehrenberg is one of the most commonly reported taxa in floristic and ecological works from around the world. Yet, the images published for this taxon reveal that its identity is not well defined and that several morphological variants have been ascribed to it. The present manuscript includes a preliminary analysis of the literature and type material housed in the Ehrenberg Collection, Berlin, Germany. The nomenclatural history of the taxon, a critical examination of original drawings, and results of the examination of type material lead to the conclusion that Fragilaria pinnata (current synonym: Staurosirella pinnata (Ehrenberg) D.M.Williams & Round) is not an araphid diatom, but rather an organism with links to the genus Denticula Kützing. The history of another taxon, Staurosira pinnata Ehrenberg, was also investigated to determine its actual relationship to Fragilaria pinnata since recent publications present these two names as synonyms. Staurosira pinnata also has an entangled history and a dubious current concept. Although, we have not examined type material for this taxon yet, based on examination of the nomenclatural history and original drawings, we were able to conclude that it is indeed an araphid diatom, but that the current synonymy with Fragilaria pinnata is incorrect.