Subgenus: Eucosmarium
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.
Subgenus: Cell body dorso-ventrally compressed, with strong constriction at the center, transverse section elliptical or diamond-shaped (Illustrations of The Japanese Fresh-water Algae, 1977).
Section Tetridium: Cell surface granulated, gnarled or spiny; semicells rectangular 
C. margaritatum
13 samples
45-81 μm
2001-2006
C. quadrum
9 samples
52-100 μm
2000-2008
C. latifrons
2 samples
54-56 μm
Japan, 2004
Cosmarium sp.
55 x 47 μm
Japan, 1999
C. ozense
22 x 18 μm
Hiroshima, 1998
C. ozense
18 x 17 μm
Iroha-numa
Zao mountain
Yamagata, 2006
C. ozense
μm
Kareki-numa
Nikko
Tochigi, 2008
