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 Globuliferae: Cell surface smooth or dotted or punctated; semicells circular, ellipsoidal, reniform 
C. contractum
30 samples
16-48 μm
1999-2008
C. depressum
14 samples
22-30 μm
1999-2008
C. bioculatum
8 samples
13-24 μm
1999-2008
C. inconspicuum
2 samples
12.5 x 10 μm
2003-2004
C. tithophorum
20 x 18 μm
Uruguay, 1999
C. pseudoprotuberans
f. minor ?
24 x 18 μm
Koigakubo marsh
Tessei-cho
Okayama, 2004
C. pseudoprotuberans
f. minor ?
21 x 15 μm
Shimo-no-koike
Shiga highland
Nagano, 2004
Cosmarium sp. ?
46 x 38 μm
3 pyrenoids ?
Myoyo
Fukiage
Saitama, 2004
Cosmarium sp. ?
55 x 40 μm
Kanayama p.
Kiyose
Tokyo, 2006
Cosmarium sp.
71 x 60 μm
Miyatoko m.5
Minami-Aizu
Fukushima, 2008
