Valves dorsiventral (asymmetric to the apical axis), often curved (lunate), symmetrical to the transapical axis. Dorsal margin convex, smooth or undulate, ventral margin straight or concave; poles rounded to capitate. Frustules almost quadrate or rectangular in girdle view, often lying in girdle view. Sometimes spines along the valve face/mantle junction.
Striae uniseriate with small, round areolae without occlusions, parallel to radiate, sometimes irregularly spaced. Narrow sternum near the edge of the valve face.
Raphe slits short and restricted to the areas near each pole. Raphe not coincident with the sternum, but to one side and lying mainly on the mantle, curved slightly or strongly onto the valve face near the apices with often conspicuous terminal fissures which curve towards the dorsal margin or back towards the valve centre. Raphe often only visible in girdle view.
Usually one rimoportula per valve near the pole, at opposite ends in epi- and hypovalves.
Several open girdle bands with rows of pores.
Plastids variable: sometimes a single ventral chloroplast; or two elongate chloroplasts stretching the whole length of the cell on the ventral side of the cell, each extending also onto one of the valves; or several plate-like chloroplasts.