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.
Section [Margaritiferae]: Cell surface granulated, gnarled or spiny; semicells pyramidal with flattened apex, scalariform (Illustrations of The Japanese Fresh-water Algae, 1977).
Cosmarium tetraophthalmum Brébisson: Cell body 90-(106-108)-120 μm long, 60-(64.4-69)-86 μm wide, L/W = 1.5; constriction deep; semicells broad ovoidal or pyramidal in shape, covered with granules (12-14 at each side) but apical ends lack granules (Illustrations of The Japanese Fresh-water Algae, 1977).