Subgenus: lacks arm-like projections (Illustrations of The Japanese Fresh-water Algae, 1977)
Species [var. senarium ]: Cell body 21-31 μm long (excluding arms), 29-36 μm long (including arms), 27-34 μm wide (including arms), isthmus 8.4-12 μm wide.
[ var. simplex Hirano ]: Smaller than var. senarium, 23 μm (excluding arms), 44 μm (including arms), 43 μm wide (including arms), isthmus 7 μm wide; projections at posterior side margins of semicells are simple spines (not dichotomous).
[ var. nigrae-silvae Schmidle ]: 2 ring of dented structures surrounding each corner, 29 μm (including arms), 36.4 μm wide, isthmus 12.5 μm wide (Illustrations of The Japanese Fresh-water Algae, 1977)

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var. senarium: Arm-like projections present at lateral side of each semicell
v. senarium 33 x 24 μm Hiroshima, 1998 Movies	v. senarium 28 x 23 μm Hiroshima, 1998 Movies	v. senarium or v. simplex ? 25 x 24 μm Iroha-numa Zao mountain Yamagata, 2006	v. senarium 25, 27 x μm Kashibaru m.II Karatsu Saga, 2008
Similar species -->> S. forficulatum;
var. nigrae-silvae: Arm-like projections short, surrounded with small spines
v. nigrae-silvae ? 31 x 28 μm Ushidome-ike Norikura highland Nagano, 2005	v. nigrae-silvae μm Numappara m.1 Nasu-shiobara Tochigi, 2007
var. simplex: Spiny projections present at ventral side of each semicell
v. simplex ? ? x 22 μm Watasuge marsh Tsugaike highland Nagano, 2005	v. simplex ? ? x 24 μm Iroha-numa 1 Zao mountain Yamagata, 2006	v. simplex ? 21 x 18 μm J Skunk Cabbage m. Tsugaike highland Nagano, 2006	v. simplex ? μm Kuroyachi marsh 2 Hachimantai Iwate, 2007
v. simplex μm Kashibaru m.II Karatsu Saga, 2008

Similar species -->> S. forficulatum (larger than S. senarium; 40-48 μm excluding arms);

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