Valve mantle not distinctive in many species.
Ornamentation between the central area and the marginal area of the valve face differs. Areolae present or absent in the valve centre, scattered in the valve centre of some species, grouped and forming lines in the outer region of the valve face. Central area flat or tangentially undulate.
A ring of marginal fultoportulae with a single rimoportula positioned on a costa.
Many disc-shaped plastids.
Cells solitary or forming filamentous colonies.
Produce chitin-threats through the fultoportulae to reduce sinking rate.