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 Cristatae: Cell surface granulated, gnarled or spiny; semicells semicircular, semi-elliptical
All margins granulated, gnarled or spiny
C. heterodentatum 2 samples 114 x 72 μm 2004-2008	C. intermedium ? or C. margaritiferum ? 6 samples 52-65 μm 2001-2003	C. margispinatum var. cornutum 60 x 48 μm Hiroshima, 1998 Movies	C. echinodermum 36 x 37 μm USA, 1999
C. quadrifarium 32 samples 41-60 μm 2003-2008	Cosmarium sp.5 14 x 13 μm USA, 1999
Margins crenated
C. nasutum 32 x 29 μm Saitama, 1999	C. caelatum v. spectabile 48 x 40 μm Shimizu-daira Nasu Tochigi, 2005

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