Frustules small, often less than 15 µm long and less than 5 µm wide, elliptic in valve view with rounded ends, rectangular in girdle view.
Axial area variable, can be wide; the central area is rounded, rhombic or an acute-angled subfascia.
Externally raphe straight or slightly curved with unilaterally straight or slightly deflected expanded proximal ends, distal raphe fissures deflected in the opposite direction.
Internally a thickened sternum can be conspicuous in LM; raphe slits terminate in helictoglossae; terminal and distal raphe ends are often conspicuous in LM.
Striae uniseriate, composed of rounded areolae on the valve face, sometimes slit-like on the mantle; areoale occluded externally by cribra.
Cingulum composed of similar narrow bands without perforations.
Cells contain two chloroplasts with a pyrenoid.