Cells isopolar, bilaterally symmetrical, valves linear-lanceolate or elliptical.
The striae are completely covered externally by a conopeum, which is perforated by a multitude of tiny pores. The conopeum is supported along the centre of the valve by the narrow longitudinal ribs adjacent to the raphe, appearing in LM as a hyaline ‘lyre’.
Window-like openings are present on the valve surface on both sides of the distal raphe fissures at the poles where the conopeum has not merged with a fold at the valve face/mantle junction.
Plastids are H-shaped with two plates lying against the girdle.