Frustules curved in girdle view; valves heteropolar or isopolar, linear, linear-elliptical or lanceolate in valve view, if heteropolar then with rounded head pole and acute foot pole.
Frustules heterovalvar, one valve having a full raphe system, the other with short raphe slits at the two poles; valves heteropolar or isopolar and bilaterally symmetrical; cells usually lying in girdle view. Isolated valves lie in valve or girdle view.
Striae uniseriate (with occasional biseriate striae), parallel or slightly radial; areolae linear, with their long axes at right angles to the striae, occluded by fine pore plates (hymenes) close to their inner apertures. At the base-pole there is an area (pore field) of more densely packed pores, through which a stalk is secreted.
Axial area narrow, widening towards the centre. Central area not clearly differentiated from the axial area.
Raphe straight, with straight, expanded endings at the centre externally, hooked endings at the centre internally, and hooked terminal fissures.
Girdle composed of open bands, each with a single row of areolae.
One complex chloroplast per cell, with its centre against one side of the girdle and containing a flat pyrenoid, from which lobes extend out beneath the valves to the opposite side of the girdle, leaving the raphe more or less free.