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 cylindrical or dumbbell-shaped, dorso-ventrally not compressed, transverse section circular (Illustrations of The Japanese Fresh-water Algae, 1977).

Section Actinotaenium: Cell body cylindrical, slightly constricted at the center, cell surface smooth or dotted or punctated
Species moved to genus Actinotaenium
C. clevei a large variety ? or C. turgidum ? 200 x 77 μm Takamatsu Kagawa, 2002	Cosmarium sp. ? C. clevei ? or Tetmemorus ? 51 x 20 μm Imou bog Toyohashi Aichi, 2004	C. curtum ? 5 samples 40-46 x 19-22 μm 2002-2006	C. diplosporum 6 samples 31-43 μm 2003-2007
C. australe 10 samples 69-92 μm 1999-2008	C. capax 4 samples 46-126 μm 1998-2006	C. cucurbitinum 7 samples 45-70 x 21-29 μm L/W=2.1-2.4 2003-2007	C. cucurbita 13 samples 30-59 x 17-24 μm 1999-2008
C. globosum 14 samples 30-36 x μm 2004-2007	Cosmarium sp. 30 x 21 μm single pyrenoid Saitama, 2001
???
C. alpestre 9 samples 51-77 x 39-57 μm 1999-2008	C. westii ? 5 samples 62-71 x 36-40 μm 2003-2008	C. oblongum 24 samples 40-63 x 20-22 μm 2004-2008	C. pseudarctoum 4 samples 16-18 x μm 2000-2006
Species remained in Cosmarium
C. thwaitesii 27 samples 29-44 x 16-24 μm 2004-2008	C. parvulum 6 samples 28-48 x 12-19 μm single pyrenoid 2004-2007

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