Genus: Variable in shape and in size; mostly longer than wide; strong constriction at center; semicell pyramid-shaped, apically flattened with indentation (or notch, some species lack this notch); laterally straight or wavy or indented; one chloroplast in each semicell; one pyrenoid for each chloroplast (Illustrations of The Japanese Fresh-water Algae, 1977).
Subgenus: Entomum (Apical area deeply indented or notched)
Section: Incisa (Cell surface smooth; apical area without spiny projections)
Species [var. ansatum ]: Cell body 87-95 μm long, 39-42 μm wide, L/W=2, constriction 11-14 μm wide (Illustrations of The Japanese Fresh-water Algae, 1977).