Cells vary in shape, often highly elongated, but some are nearly isodiametric. Valve outlines exhibit significant variability, including linear, lanceolate, elliptical, or sigmoid shapes, with or without a central constriction, and poles may be rounded or rostrate to capitate. The arrangement of cells in valve view or girdle view depends on the species. They are generally isovalvar, isopolar, and relatively symmetrical concerning the apical axis but structurally asymmetrical due to the off-center placement of the raphe system. The raphe systems on the two valves may be on the same side or opposite sides of the cell. Striae are typically uniseriate, parallel, with round or transapically elliptical areolae that may be occluded by hymenes and occasionally cribra. The raphe system is usually fibulate and eccentric, often visible by the presence of fibulae. Central raphe endings may be present or absent, and fibulae are rib-like, usually small, but sometimes extending across the valve. Typically, there are two chloroplasts per cell, positioned fore and aft, with occasional exceptions in some marine species. Chloroplasts are usually simple and pressed against one of the valves or the girdle.