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 Microcosmarium: Cell surface smooth or dotted or punctated; semicells polygonal 
C. subrectangulare
or C. sexangulare ?
37 x μm
Aquatic botanical g.
Sawara
Chiba, 2004
C. sexangulare
2 samples
var. sexangulare
38-41 x μm
2004-2005
C. laeve
44 samples
18-39 μm
1999-2008
C. scopulorum
8 samples
21-29 μm
1999-2005
C. impressulum
12 samples
18.5-32 μm
1999-2005
C. meneghinii
4 samples
18 x 14 μm
Shimo-no-koike
Shiga highland
Nagano, 2004
C. regnellii
7 samples
15-25 μm
2001-2007
C. repandum
6 samples
15.5-21 μm
2004-2006
C. clepsydra
24 x 18 μm
Umi-no-nakamichi P.
Fukuoka, 2002
C. clepsydra
16 x 14.5 μm
Aquatic botanical g.
Sawara
Chiba, 2004
C. braunii ??
or C. scopulorum ?
26 x 19 μm
Uruguay, 1999
C. abbreviatum
9 samples
12-19 x 14-18 μm
2000-2007
C. abbreviatum
7 samples
12-19 x 14-18 μm
2000-2007
Cosmarium sp.
or C. hammeri ?
47 x 29 μm
Riverside green l.
Sawara
Chiba, 2004
