Valves twisted about the apical axis, frustules often lying in girdle view and then bilobate in shape.
Striae usually bi- or multiseriate with small, round areola openings occluded by hymens; sometimes striae are just a delicate, porous membrane; internally transapical ribs are sometimes present.
Valve with high, narrow keel which becomes lower or disappears in the valve centre.
Raphe system fibulate or the side of the keel is fused beneath the raphe, raphe positioned on outer edge of keel and terminating close to each pole; fibulae short bar-like struts on the transapical ribs, the fibulae may occur at many levels beneath the raphe where the keel is high.
Many girdle bands.
One or two plastids.