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 (var. sportella): Cell body 34-42-45 μm long, 30-42-45 μm wide, isthmus 10-13-14.5 μm wide, large granules present at the center of the semicells; apex flat; (Illustrations of The Japanese Fresh-water Algae, 1977).

Similar species -- >> C. vexatum; C. botrytis;

31 μm 63 μm 94 μm; x 640

Cosmarium botrytis var. subtumidum Wittrock: Cell body 52-(56)-80 μm long, 43-(51)-65 μm wide, isthmus 10-(14)-15 μm wide, (Illustrations of The Japanese Fresh-water Algae, 1977).

* Cosmarium sportella Brébisson var. subnudum: Cell body (42)-51 μm long, (33.6)-41 μm wide, isthmus 11-(14.5) μm wide, apex slightly depressed; (Illustrations of The Japanese Fresh-water Algae, 1977).