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Abstract
Light and scanning electron microscopical studies of the rare freshwater taxon Oestrupia zachariasii from Lake Hovsgol, Mongolia, clearly indicate that this taxon does not have features characteristic of Oestrupia, but belongs within the genus Biremis as B. zachariasii (Reichelt) Edlund, Andresen & Soninkhishig comb. nov. Biremis has internal poroid cribral plates covering marginal chambered striae that open externally by two foramina, one on the valve face and one on the valve mantle. In contrast, Oestrupia is characterized by Pinnularia/Caloneis-type alveolate striae opening internally by large internal foramina and interrupted by longitudinal costae. Biremis zachariasii also has a longitudinal row of occluded areolae on each side of the axial area, similar to some specimens of B. ambigua. Oestrupia zachariasii var. undulata is separable from the nominate variety as a shape series and is similarly proposed for transfer to Biremis as B. undulata(Schulz) Andresen, Edlund & Soninkhishig comb, et stat. nov. As currently understood, Biremis includes both marine and freshwater taxa, whereas the removal of B. zachariasii and B. undulata leaves Oestrupia as a primarily marine/brackish genus.
