Eucocconeis laevis

Frustules are often strongly bent in the girdle view with concave raphe valve and convex rapheless valve. Valves are elliptical-lanceolate with slightly drawn-out ends. The raphe valve has a narrow axial area and an asymmetrical central area, which is usually expanded to the valve margin on one side. The raphe is straight or slightly sigmoid with terminal ends sharply turned to opposite directions. Rapheless valve has a narrow linear or slightly sigmoid axial area and a large central area of variable shape, often asymmetrical an expanded to one or both valve margins. Sometimes a shallow depression is visible on one side of the central area. The striae are radiate, 25-35 in 10 μm.

Note that this concept of E. laevis is wide and includes types of E. quadratarea (Ostrup) Lange-Bertalot in Lange-Bertalot and Genkal, E. diluviana (Hustedt) Lange-Bertalot in Lange-Bertalot and Genkal, and E. lapponica Hustedt. North American researchers have referred this taxon to Achnanthes lapponica var. ninckei (Guerm. and Mang.) Reimer [Syn.: Eucocconeis lapponica var. ninckei (Guerm. and Mang.) Stoermer and Yang 1969] based on its treatment in Patrick and Reimer (1966). Further investigation is necessary to establish whether these taxa represent separate species.