Species [ var. orbiculare ]: 33.6-44.8 μm long, 28-39 μm wide, isthmus 7-11 μm wide; semicells nearly semicircular in side view; cell wall dotted.
[ var. ralfsii W. et G.S. West ]: 31-41 μm long, 22-36 μm wide, isthmus 7-11 μm wide; semicells close to triangular in side view.
[ var. messikommeri Hirano ]: 21 μm long, 22 μm wide, isthmus 6.5 μm wide; semicells longitudinally depressed, semicircular or spindle-shaped in side view; bay opened.
[ f. subangulata Messikommer ]: 39 μm long, 29.4 μm wide, isthmus 12.6 μm wide; semicells conical in side view; bay opened
[ var. depressum Roy et Bisset ]: 19.6-29.4 μm long, 19.6-28 μm wide, isthmus 7-8.4 μm wide; semicells longitudinally depressed semicircular in side view.
[ var. hibernicum W. et G.S. West ]: Larger than var. orbiculare, 42-65 μm long, 36-56 μm wide, isthmus 9.5-20 μm wide; apex of semicells flattened (Illustrations of The Japanese Fresh-water Algae, 1977).

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var. hibernicum: Large, 42-65 x 36-56 μm
v. hibernicum 57 x 47 μm Yamaguchi Univ. Yamaguchi, 2004	v. hibernicum 53 x 47 μm Yamaguchi Univ. Yamaguchi, 2004	v. hibernicum 46 x 43 μm Yamaguchi Univ. Yamaguchi, 2004
var. orbiculare: 33.6-44.8 x 28-39 μm
S. orbiculare 40 x 36 μm Uruguay, 1999	S. orbiculare 45 x 40 μm Kaga Ishikawa, 2000	S. orbiculare strain Ikk-2 Cell division Ishikawa, 2000	S. orbiculare 43 x 45 μm Niigata, 2001
var. ralfsii: 31-41 x 22-36 μm; semicells close to triangular in side view
S. orbiculare x 31 μm Oze s.k.m. Oze-ga-hara Gunma, 2006	S. orbiculare 31 x 27 μm Miyatoko m.5 Minami-Aizu Fukushima, 2008	S. orbiculare 20 x 25 μm Naruto Chiba, 2003	S. orbiculare 29 x 24 μm Tanohara marsh Shiga highland Nagano, 2004
var. depressum: 19.6-29.4 μm long, 19.6-28 μm; semicircular; bay closed
S. orbiculare 26 x 23 μm Tanohara marsh Shiga highland Nagano, 2004	S. orbiculare 30 x 30 μm Ishinazaka Furukawa Miyagi, 2003	S. orbiculare 35 x 29 μm Sensho-ga-hara Shizukuishi Iwate, 2007	S. orbiculare 28 x 27 μm Kareki-numa Nikko Tochigi, 2008
v. depressum 24 x 22 μm Tanohara marsh Shiga highland Nagano, 2004	v. depressum 22 x 20 μm Tane-ike Togakushi highland Nagano, 2005	v. depressum 25 x 23 μm Hikage marsh Shiga highland Nagano, 2005	v. depressum x 19 μm Doman Green P. Saitama, 2001
v. depressum ? x 11 μm Biwanokage A.P. Tajima Fukushima, 2004
var. messikommeri: Small, 21 x 22 μm; semicircle/spindle-shaped; bay opened
v. messikommeri 23 x 23 μm Tanohara marsh Shiga highland Nagano, 2004	v. messikommeri 32 x 27 μm Karakemi marsh Omachi Nagano, 2006

Similar species -->> Staurastrum muticum; Spondylosium javanicum;

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