Frustules lanceolate or rhombic in valve view, with acute or capitate poles.
Frustules isovalvar, isopolar and bilaterally symmetrical; girdle narrow and so cells usually lie in valve view.
Striae uniseriate, parallel, fine (relative to the moderate size of most species); areolae small, circular, arranged in longitudinal (apical) as well as transapical rows and occluded by fine pore plates (hymenes) at their inner apertures.
Axial area narrow; internally the raphe sternum bears a thick longitudinal rib on each side of the raphe, which combines with the helictoglossa to form a characteristic beaked (‘porte-crayon’) structure at each pole and with the central nodule to form an H- or X-shaped reinforcement at the centre. Central area scarcely differentiated from the axial area or slightly expanded.
Raphe straight or slightly biarcuate, with T- or Y-shaped endings at the centre and poles externally (more rarely bent to one side at the centre).
Girdle composed of several narrow bands, at least one of which is folded into a tube along its length.
Two plate-like chloroplasts per cell, one against each side of the girdle, each with a prominent central pyrenoid.