Genus: Colonies of 4 (or 2, 8, 16) cells attached side by side, arranged linearly or zigzag; cell body elliptical or spindle or crescent in shape; terminal cells with spiny projections in many species; cell wall usually smooth, but in some species granulated or dented or ridged (Illustrations of The Japanese Fresh-water Algae, 1977).
Species [ var. arumatus ]: Colonies of 2, 4, 8 cells arranging linearly or slightly sigzag; cell body ovoid or elongated ellipsoidal, both ends rounded, 10-20 μm long, 3.5-8 μm wide; each cell with longitudinal ridges on both sides, ridges distinct at both ends but unclear at the center; outer cells with spiny projections (8-15 μm long) at both ends (Illustrations of The Japanese Fresh-water Algae, 1977).

Longitudinal ridges present in all cells but obsucured at central region
v. armatus 11 x 3, 12 x 4 μm Tane-ike Togakushi highland Nagano, 2005	v. armatus ? μm Maimeno-ike Norikura highland Nagano, 2007	v. armatus or v. boglariensis ? cell μm O-ike park Yabuki Fukushima, 2003	v. armatus or v. boglariensis f. crassicaudatus ? cell 14 x 3.5 μm spines 14 μm Ichikawa Chiba, 2001
v. armatus or v. boglariensis f. crassicaudatus ? cell μm O-ike park Yabuki Fukushima, 2003	v. armatus cell 12 x 5.5 μm Shiba river near Owada Park Saitama, 2005	v. longispina stock Tnk-2 cell 11 x 3-5 μm Nishi-nasuno Tochigi, 2001	v. longispina stock Tnk-2 cell 11 x 6 μm Nishi-nasuno Tochigi, 2001
Outer cells with only one spiny projection
v. bicaudatus ? cell μm L/W= Kazo Saitama, 2003	v. bicaudatus cell 18 x 5-6 μm spines 18 μm Shiba river near Owada Park Saitama, 2005	v. bicaudatus cell 12 x 4.5 μm spines 9 μm Sue-machi Fukuoka, 2001	v. bicaudatus 9 x 2.5 μm spines 8 μm Umi-no-nakamichi Fukuoka, 2001
v. bicaudatus ? cell μm L/W= Akebono-yama A.P. Kashiwa Chiba, 2002
Spiny projections short, curving inward
v. chodatii ? inner 11 x 4 μm outer 9 x 4 μm Abukuma river Fukushima, 2005	v. chodatii ? inner μm outer μm Itakura Gunma, 2003	v. chodatii ? inner 13 x 5 μm outer 10 x 4 μm Umi-no-nakamichi Tochigi, 2001
Misc.
S. armatus ? μm Uruguay, 1999	S. armatus ? μm Kayoicho-ike Fukuoka, 2001	S. armatus ? μm Iruma river Sayama Saitama, 2004	S. armatus ? 12-13 x 4 μm Hasu-ike Shiga highland Nagano, 2005

Similar species -->> S. quadricauda; S. longispina;

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