Cells solitary. Frustules isopolar, bilaterally symmetrical. Cells lie in valve or girdle view and isolated valves always in valve view. Valves bilaterally symmetrical, linear to linear-lanceolate (rarely lanceolate, in the smallest specimens), with subrostrate or cuneate poles (becoming apparently rounded in small lanceolate valves). Striae clearly visible in LM and stria pores often also resolvable. Raphe system fibulate (the raphe itself is impossible to detect in LM), marginal. Fibulae small, dot-like or ± square. Central pair of fibulae usually more widely separated than the others; central raphe endings present and detectable with care by a tiny pimple-like thickening of the margin. In a frustule, the raphe systems of the two valves lie on opposite sides ('nitzschioid symmetry') Two chloroplasts per cell, one towards each pole. Each chloroplast is a simple plate, which lies against one valve and one side of the girdle.