Frustules lanceolate, rhombic, elliptical, linear-lanceolate or with undulate margins in valve view, with bluntly rounded, cuneate, rostrate or capitate poles.
Frustules isovalvar, isopolar and bilaterally symmetrical; cells lying in valve and in girdle view.
Striae uniseriate, slightly or strongly radial, coarsely areolate (relative to the small to medium size of most species), interrupted at the junction of valve face and mantle; areolae circular or transapically elongate, occluded by fine pore plates (hymenes), which form a more or less continuous strip along the inner face of each stria. At least in some, there is a longitudinal canal in the wall of the valve at the junction of valve face and mantle.
Axial area narrow. Central area strongly developed, extending to close to the edge of the valve face in many cases, as a broad plain area (fascia); on one side (the secondary side) the area usually contains a special pore (stigma), which has a simple round aperture externally and opens internally via a lipped structure that somewhat resembles a rimoportula.
Raphe straight, curving slightly towards one side (the primary side) at the centre; central external raphe endings simple or deflected; internal central raphe endings straight; terminal fissures bent or hooked.
Girdle composed of several narrow bands. One complex chloroplast per cell, consisting of two lobed plates against the valves, connected by a bridge containing a prominent pyrenoid.