Genus: Unicellular; variable in shape; a constriction at the center of the cell body;
mostly longer than wide; flattened; each semicell hemispherical, spherical, ellipsoidal,
rectangular, pyramidal or kidney-shaped; no apical indentation
Subgenus [Eucosmarium]: Cell body dorso-ventrally compressed, with strong constriction at the center, transverse section elliptical or diamond-shaped.
Section [Quadratiformes]: Cell surface smooth; semicells rectangular (Illustrations of The Japanese Fresh-water Algae, 1977).