Cells solitary, lanceolate to lanceolate-elliptical with rostrate to capitate poles.
Stria mostly parallel, equidistant, uniseriate. Areolae aligned in longitudinal and transverse rows, forming straight lines parallel to raphe sternum. Areola openings small, round, elliptical or linear, separated by frets. In some species areolae open externally into grooves between longitudinal ribs. Areolae occluded by hymenes near their inner apertures.
Raphe sternum narrow, not or slightly elevated externally, thickened internally (more or less equally on either side of the raphe) and without accessory ribs. Central area small or absent. Externally, proximal raphe endings expanded, slightly bent towards primary side of valve, terminal raphe fissures bent towards the secondary side and recurved. Internally, proximal raphe ends not expanded, straight or very slightly curved, distal ends terminating in small helictoglossae.
Cells polymorphic, forming internal valves during resting spore formation to withstand changes in osmotic pressure and to survive desiccation. Two types of internal valves are formed. Craticulae are reduced valves consisting of a raphe sternum and distantly spaced transverse bars, which are usually connected together at the margin; these form inside the normal valves. Heribaudii valves, the innermost valves formed inside the Craticula, have a slightly arched valve surface and possess a different striation pattern, the striae being radial rather than parallel and formed by small, round areolae which are not aligned longitudinally or transversally.
Cingulum with a series of open bands which gradually diminish in width.
Usually two, sometimes one, chloroplasts, with simple or slightly undulating margins, sometimes with indentation at centre. One chloroplast against each side of the girdle, moving onto valve before cell division. Pyrenoids difficult to observe, not long bars (contrast Navicula). Often with two prominent volutin granules, one in each apical vacuole.