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
(Illustrations of The Japanese Fresh-water Algae, 1977).
Cosmarium ozense Hirano: Cell body 17-18 μm long, 18-18.5 μm wide, isthmus 7 μm wide; semicells rectangular (Illustrations of the Fresh-water Algae, 1977).