Species [ var. connatum ]: Cell body 22-24 μm long, 19.6-25 μm wide (excluding spines), isthmus 7-9 μm wide; spines short; semicells inverted semicircle in shape, apex flat or slightly convex.
[ var. pseudamericanum Grönblad ]: Cell body larger than v. connatum 39-45 μm long, 25-36.45 μm wide (excluding spines), isthmus 5.6-11 μm wide; corners of semicells protruded upward making its apex concave.
[ var. rectangulum Roy et Bisset ]: Cell body 14-20 μm long, 14.7-20 μm wide (without spines), isthmus 3.5-5.6 μm wide; side margins of semicells not convex; spines longer and thicker than v. connatum (Illustrations of The Japanese Fresh-water Algae, 1977)

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var. connatum: Semicells inverted half-circle; 22-24 μm long (excluding spines)
v. connatum 25 x 25 μm Onnenai Kushiro marsh Hokkaido, 2005	v. connatum 23 x 23 μm Kejo-numa 1 Osaki Miyagi, 2006	v. connatum or S. erlangense ? 39 x 29 μm USA, 1999	v. connatum ? or S. erlangense ? 32 x 32 μm Kasuga Fukuoka, 2001
v. connatum or S. megacanthum ? 33 x μm Ko-numa 2 Minakami Gunma, 2008	v. connatum or S. megacanthum ? 33 x μm pond near R272 Minakami Gunma, 2008
var. rectangulum: Semicells inverted triangle; 14-20 μm long (excluding spines)
v. rectangulum 33 x 30 μm Kabuto-yama Nishi-no-miya Hyogo, 2004	v. rectangulum 26 x 20 μm Japan, 1998	v. rectangulum 28 x 28 μm water reservoir near Yamaguchi Univ. Yamaguchi, 2004	v. rectangulum 24 x 20 μm Uruguay, 1999
v. rectangulum 20 x 18 μm Mida-ga-hara Gassan Yamagata, 2005	v. rectangulum 19 x 18 μm Futayo-no-ike Haguro mountain Yamagata, 2005	v. rectangulum 19 x 18 μm Maimeno-ike Norikura highland Nagano, 2006	v. rectangulum 23 x 22 μm Kejo-numa 2 Osaki Miyagi, 2006
v. rectangulum 20 x 22 μm Kashibaru m.5 Karatsu Saga, 2006	v. rectangulum 22 x 19 μm Ohnuma Nasu-shiobara Tochigi, 2007	v. rectangulum μm near Omine mt. Minakami Gunma, 2007	v. rectangulum μm Kannon-numa 10 Shimogo Fukushima, 2008
v. rectangulum μm Kannon-numa 11 Shimogo Fukushima, 2008	v. rectangulum μm Kareki-numa Nikko Tochigi, 2008
var. pseudamericanum: Apex depressed; corners protruding; 39-45 μm long (excluding spines)
v. pseudamericanum or S. curvatum ? μm Numappara marsh Kuroiso Tochigi, 2003	v. pseudamericanum or S. curvatum ? 21 x 15 μm 18 x 16 μm Tane-ike Togakushi highland Nagano, 2005	v. pseudamericanum or S. curvatum ? 26 x 26 μm Karakemi marsh Omachi Nagano, 2006
v. pseudamericanum or S. curvatum ? 18 x 19 μm Iroha-numa Zao mountain Yamagata, 2005	v. rectangulum or S. curvatum ? 30 x 23 μm Iroha-numa Zao mountain Yamagata, 2006	v. rectangulum or S. curvatum ? μm Iroha-numa Zao mountain Yamagata, 2008	v. rectangulum or S. curvatum ? μm near Omine-numa Minakami Gunma, 2008
v. rectangulum ? or S. curvatum ? ? x 20 μm Kashibaru marsh 5 Karatsu Saga, 2006	v. rectangulum or S. curvatum ? μm Kashibaru m.4 Karatsu Saga, 2008

Similar species -->> S. lunatum; S. apiculatum;

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