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 Pyramidatae: Cell surface smooth or dotted or punctated; semicells pyramidal with flattened apex or trapezoid
All margins smooth (not wavy)
C. granatum 16 samples 25-41 μm 1998-2007	C. pyramidatum 8 samples 77-84 μm 1999-2007	C. pseudopyramidatum 12 samples 49-71 μm 2003-2008	Cosmarium sp. 41 x 29 μm Kami-hashi Namegawa Saitama, 2003
Cosmarium sp. 32 x 21 μm Kurome river Asaka Saitama, 2004	C. subtumidum 27 samples 19-49 μm 1998-2008	C. succisum 5 samples 10-14 μm 2001-2004	C. trilobulatum 1 sample 35 x 23 μm Omine numa Gunma, 2003
C. dorsitruncatum μm Ohdo-ike Chuo-ku Chiba, 2007	C. dorsitruncatum ? 46 x 50 μm Kejo-numa Furukawa Miyagi, 2003	C. dorsitruncatum ? 33 x 30 μm Kejo-numa 1 Osaki (=Furukawa) Miyagi, 2006	C. dorsitruncatum ? μm near Omine mt. Minakami Gunma, 2007
C. synduorhabdum ? 30 x 26 μm Uruguay, 1999	C. hammeri v. protuberans 7 samples μm 1998-2008	C. incertum ? μm Ohyachi 2 Hachimantai Akita, 2007
Margins wavy
C. visbyense 2 samples 32-45 μm single pyrenoid 1998-2004	C. notabile ? Margaritiferae ? 5 samples 21-42 μm 1997-2004 Movies	C. suborthogonum ? 23 x 21 μm Yasuji-kawa Sawara Chiba, 2004

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