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). Species: Cell body rectangular, 53-90 μm long, 47.6-85 μm wide; isthmus 19-30 μm wide; semicells subrectangular, with rounded corners; side view nearly circular; apical view elongated ellipsoidal; marginal granules 34-37 in number (Illustrations of the Japanese fresh-water algae, 1977).

31 μm 63 μm 94 μm; x 640

Cosmarium margaritatum (Lundell) Roy et Bisset var. margaritatum: Cell body 78-90 μm long, 70-72 μm wide, L/W=1.20-1.25, isthmus 22-25 μm wide; semicells subrectangular, basal angles rounded, lateral margins convex, anical angles broadly rounded, apex truncate and flat; cell wall regularly granulated, between the granules finely punctated; side view of the semicells subcircular, vertical view oblong-elliptic; zygospores spherical or slightly depressed, with yellowish smooth wall, surrounded by mucilagenous sheath, 44.8-47.6 μm in diam. (Photomicrographs of the Freshwater Algae, vol. 3, 1985).

Cosmarium margaritatum f. minor (Boldt) Roy et Bisset: Cell body rectangular, 58-59 μm long, 54 μm wide; isthmus 18-19 μm wide (Illustrations of the Japanese fresh-water algae, 1977).

Cosmarium quadrum Lundell var. quadrum: Cell body rectangular, 53-90 μm long, 47.6-85 μm wide; isthmus 19-30 μm wide; semicells subrectangular, with rounded corners; side view nearly circular; apical view elongated ellipsoidal; marginal granules 34-37 in number (Illustrations of the Japanese fresh-water algae, 1977).

C. quadrum var. minus Nordstedt: Cell body 45-51 μm long, 40-47 μm wide, constriction 17-19 μm wide (Illustrations of The Japanese Fresh-water Algae, 1977).