Platessa oblongella

Valves are elliptical-lanceolate to linear-elliptical. The raphe valve has a narrow linear axial area and a transversely rectangular or bow tie-shaped central area. The raphe is straight, without terminal fissures. The proximal and distal raphe ends are slightly expanded. The proximal raphe ends curve toward opposite sides internally. Striae on raphe valve are slightly radiate in the valve center and strongly radiate and curved at apices, 24-30 in 10 µm. The areolae openings are small, round. The rapheless valve has a narrow lanceolate axial area. The central area is usually not differentiated from the axial area, but sometimes two middle striae on one side of the valve are more distant from each than other striae. Striae on rapheless valve are broader than those on the raphe valve, punctate, parallel in the center and radiate at apices, 9-12 in 10 µm. Shortened marginal striae are often present. The openings of the areolae are apically elongated, internally areolae open into a shallow chamber.