Ardissoneales Round 1990, emend. Lobban & Ashworth

Description.

Highly elongate valves with development from a bifacial annulus (not always visible) and lacking rimoportulae and apical pore fields (ocelli/ocellulimbi). Frustules often morphologically heteropolar in valve view, girdle view, or both, attached to substrata by mucilage stalks. Valves with simple areolae in uniseriate striae, internal transapical costae more or less developed. Valvocopula and copula closed, fimbriate internal margins. Pleura (when present) narrow, fimbriate inner margin, with larger apical caps.