Pyxidiculacompressa: Diatom or dinoflagellate?

The taxonomic position of Pyxidicula compressa, currently the nomenclatural basionym of two unrelated taxa (a diatom and a dinoflagellate), is clarified. Evidence is presented that the protologue of the species clearly represents a diatom, which is then correctly known as Tryblionella compressa. In the absence of original specimens, the species is lectotypified with illustrations from the protologue. It is proposed that the name Prorocentrum bidensbe adopted for the dinoflagellate presently known as Prorocentrum compressum.