Valves, linear, linear-lanceolate, rhombic or with undulate valve margins, with rounded, cuneate or protracted ends, the latter can be rostrate, subcapitate or capitate.
Isopolar, some some species heteropolar, bilaterally symmetrical.
External valve surface can be ornament with warts. A siliceous ridge or hyaline area along the valve margin separates valve face and mantle (visible in the SEM).
Narrow, linear axial area, central area varies considerably in size, indistinct, rounded, apically elongate, or rhombic.
Raphe located on both valves, externally with longitudinal ribs, distal fissures sometime ending in T-shaped depression, over which a silica flap can extend.
Striae uniseriate, punctate, with transapically elongated areolae, sometimes forming longitudinal, undulating lines.
Cells possess one plastid.