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Abstract
The marine diatom Plagiogrammopsis mediaequatus sp. nov. (Cymatosiraceae, Bacillariophyta) is described from various localities around Britain. Its frustule morphology is compared with that of the type species of the genus, Plagiogrammopsis vanheurckii (Grun.) Hasle, von Stosch & Syvertsen, with Plagiogramma leve (Greg.) Ralfs and with Brockmanniella brockmannii (Hust.) Hasle, von Stosch & Syvertsen. The major differences from the type species lie in the lack of a pseudoseptum, in the presence of a large wing on the tip of each pilus, and in the less-pronounced terminal elevations. All of these features are visible with the light microscope. The vela are also simpler than those of the type species. New detail of the pili and marginal spines of P. vanheurckii is also presented.
