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).

Subgenus: Dysphinctium (Cell body cylindrical with slight constriction at center)
Actinotaenium: Cell surface smooth
C. alpestre x 640	Cosmarium sp.	C. cucurbita x 640	�@
Subgenus: Eucosmarium (Cell body flat with strong constriction at center)
Globuliferae: Semicells circular, ellipsoid, reniform; Cell surface smooth
C. depressum var. minor	C. tithophorum
Ornatae: Semicells circular, ellipsoid, reniform; Cell surface granulated or spiny
C. logiense f. expansa x 640	Cosmarium sp. x 640
Cyclidium: Semicells semi-circular, -ellipsoid; Cell surface smooth
Cosmarium sp. x 640	Cosmarium sp. x 640	Cosmarium sp. x 640
Cristatae: Semicells semi-circular, -ellipsoid; Cell surface granulated or spiny
Cosmarium sp. x 640
Pyramidatae: Semicells pyramidal, ??? ; Cell surface smooth
C. pyramidatum x 640	Cosmarium sp. x 640
Margaritiferae: Semicells pyramidal, ??? ; Cell surface granulated or spiny
Cosmarium sp. x 640	C. subspeciosum x 640	C. formosulum x 640	Cosmarium sp. x 640
Cosmarium sp. x 640	C. orthostichum x 640
Tetridium: Semicells rectangular; Cell surface smooth
C. ornatum x 640
Microcosmarium: Semicells polygonal; Cell surface smooth
Cosmarium sp. x 640