Frustules lanceolate, linear or linear-elliptical in valve view, with rounded, acute or rostrate poles.
Frustules isovalvar, isopolar and bilaterally symmetrical; cells usually lying in valve view.
Striae uniseriate, parallel or radial; areolae linear, with their long axes at right angles to the striae and aligned to form straight or curved longitudinal lines, occluded by fine pore plates (hymenes) at their inner apertures.
Axial area narrow; internally the raphe sternum usually bears a continuous longitudinal rib on one side of the raphe (the primary side). Central area often developed as a plain circular or rectangular area; it is thickened internally and links with the longitudinal rib borne internally by the raphe sternum.
Raphe straight, straight or hooked endings at the centre externally, simple straight endings at the centre internally, and hooked terminal fissures.
Girdle composed of plain open bands, of which the first is by far the widest.
Two plate-like chloroplasts per cell, one against each side of the girdle, each usually with a single long bar-like pyrenoid (visible in girdle view only).