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 Margaritiferae: Cell surface granulated, gnarled or spiny; semicells pyramidal with flattened apex or trapezoid
C. pseudomagnificum 7 samples 76-98 x 55-68 μm 2003-2008	C. decoratum 9 samples 70-84 μm 2003-2007	C. subspeciosum 5 samples var. subspeciosum 35-36 x 29-30 μm 2004-2005	C. subspeciosum 1 sample var. validius 89 x 63 μm Uruguay, 1999
Cosmarium sp. 160 x 108 μm Uruguay, 1999	Cosmarium sp.1 111 x 66 μm L/W=1.7 USA, 1999 Movies	C. tetraophthalmum 88 x 64 μm L/W=1.4 USA, 1999	C. binum 14 samples 47-62 μm 1998-2008 Movies
C. ochthodes 3 samples 72-82 x 55-59 μm 2005-2006	C. margaritiferum 39 samples 49-81 μm 1999-2008	C. vexatum 21 samples 31-56 μm 1998-2006	C. botrytis var. subtumidum 25 samples 59-77 μm 1999-2008
C. raciborskii 56 x 56 μm near City Hall Fujimi Saitama, 2004	C. turpinii var. eximium 65 x 52 μm near Karakemi m. Omachi Nagano, 2005	C. turpinii var. eximium 45 x 39 μm Tsukamoto Sakura-ku Saitama, 2004	C. sportella 8 samples 38-48 μm 1998-2004 Movies
C. quinarium 2 samples 39-45 μm 1999-2006	C. trachypleurum var. cornutum 49 x 40 μm Nagori-ga-hara Sukawa highland Iwate, 2007
C. formosulum 33 samples 21-41 μm 1999-2008 Movies	C. furcatospermum v. furcatospermum 9 samples 16-26 μm 2001-2008	C. subcostatum 4 samples 17-33 x μm 2003-2006
C. polonicum ? 20 x 17 μm Univ. A.V.M. Obihiro Hokkaido, 2005	C. polonicum ? 20.5 x 16.5 μm Water reservoir Iiyama Nagano, 2008	C. notabile ? Pyramidatae ? 7 samples 21-42 μm 1997-2005 Movies	C. subcrenatum var. subcrenatum 40 x 30 μm Ikenokochi marsh Tsuruga Fukui, 2004
C. subcrenatum var. subcrenatum ? 35 x 25 μm Iyari marsh Omachi Nagano, 2006	C. subalatum μm Omine-numa Tsukiyono Gunma, 2003	C. orthostichum v. subpolonica 31 x 28 μm Uruguay, 1999	C. punctulatum 10 samples var. punctulatum 28-45 μm 2004-2006
C. sikhimense ? 46 x 31 μm Koigakubo marsh Tessei-cho Okayama, 2004

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