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 small, (21-29.5)-36 μm long, (15.4-18)-26 μm wide, isthmus 5-(5.6-7)-9 μm wide, L/W=1.5; both sides of semicells with four wavy margins, all waves equal in size, side view elongated ellipsoid; cell wall smooth (Illustrations of The Japanese Fresh-water Algae, 1977).

31 μm 63 μm 94 μm; x 640

Cosmarium impressulum Elfving var. impressulum: Cell body small, (21-29.5)-36 μm long, (21-29.5)-36 μm wide, isthmus (15.4-18)-26 μm wide, L/W=1.5; (Illustrations of the Japanese Fresh-water Algae, 1977).