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 [ Cyclidium ]: Cell surface smooth; semicells half circle, half ellipsoid (Illustrations of The Japanese Fresh-water Algae, 1977).
Species: Cell body large in size, 138-168 μm long, 73-90 μm wide, L/W=2; constriction 39-45 μm wide; semicells half-elliptical or ovoidal; cell wall dotted (Illustrations of the Japanese fresh-water algae, 1977).