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 Cyclidium: Cell surface smooth or dotted or punctated; semicells half circle, half ellipsoid 
C. maculatum
5 samples
121-142 μm
1998-2004
Movies 
C. pachydermum
32 samples
65-114 μm
1999-2008
C. lundellii
6 samples
45-76 μm
2001-2008
taxichondrum
3 samples
84-100 μm
2004-2008
obsoletum
26 samples
44-82 μm
1998-2008
C. circulare
var. circulare
2 samples
45-52 x μm
1999-2004
C. circulare
f. minor ?
2 sample
Uruguay, 1999
C. ralfsii
var. montanum
2 samples
60-74 x 61-67 μm
Nishi-no-miya
Hyogo, 2004
C. subauriculatum
var. subauriculatum
55 x 57 μm
near Yamaguchi Univ.
Yamaguchi, 2004
C. ocellatum
4 samples
26 x 25 μm
2004-2007
C. pandriforme ?
12 samples
56-72 x μm
2003-2007
C. subcucumis ?
71 x 39 μm
Iyari marsh
Omachi
Nagano, 2006
C. subcucumis
or C. microsphinctum ?
64 x 43 μm
Japan, 1998
Movies 
C. phaseolus
var. minor
20 x 18 μm
Hiroshima, 1998
C. maximum
var. minor
52 x 47 μm
Mashiko
Tochigi, 1999
Cosmarium sp.
or Pyramidatae
C. scopulorum ?
22 x 15 μm
Kaga
Ishikawa, 2000
Cosmarium sp.
27 x 23 μm
Komado marsh
Showa and Tajima
Fukushima, 2004
C. obtusatum ?
or C. subcostatum ?
34 x 27 μm
USA, 1999
C. petsamoense
v. simplicius
34 x 26 μm
Ichinose-enchi
Norikura highland
Nagano, 2005
