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:

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Outer cells laterally straight
f. antenniformis or var. acutus ? cell 13 x 4.5 μm L/W=2.9 Koshigaya Saitama, 2001	f. antenniformis all cells straight cell 9 x 2.5 μm L/W=3.6 Japan, 1998	f. antenniformis all cells straight cell μm L/W= Kodaira Tokyo, 2003	f. antenniformis ? cell μm L/W= Ishikiriba Shirakawa Fukushima, 2003
Cells arranging zigzag
f. alternans cells zigzag cell 28 x 10 μm L/W=2.8 Japan, 1997	f. alternans cells zigzag cell 12 x 5.9 μm L/W=2.0 Mizumoto Park Tokyo, 2001	f. alternans cells zigzag cell μm L/W= Kodaira Central Park Tokyo, 2003
Cells arranging in two rows
f. costulatus cells in two rows cell 9 x 3 μm L/W=3.0 Japan, 1998	f. costulatus cells in two rows cell 11 x 3.5 μm L/W=3.1 Japan, 1999	f. costulatus cells in two rows cell 10 x 4 μm L/W=2.5 Japan, 2000	f. costulatus cells in two rows cell 11 x 3.5 μm L/W=3.1 Kuroiso Tochigi, 2001
f. costulatus cells in two rows cell μm L/W= Kodaira Central Park Tochigi, 2003	f. costulatus cells in two rows cell μm L/W= Kodama town Saitama, 2003	f. costulatus cells in two rows cell 14 x 5 μm L/W= Kawanago Ishibashi Tochigi, 2004	f. costulatus cells in two rows cell 13-35 x 4-7 μm L/W= Iruma river Kawagoe Saitama, 2004
f. costulatus cells in two rows cell 11 x 4 μm L/W= Koma river Sakado Saitama, 2004	f. costulatus cells in two rows cell 13 x 5 μm L/W=2.6 Showa-machi g.p. Showa Saitama, 2006	f. costulatus cells in two rows cell 13-15 x 5.5 μm L/W=2.4-2.7 Toyonodai park Otone Saitama, 2006	f. costulatus cells in two rows cell 11 x 4 μm L/W=2.8 Kodai-hasu p. Gyoda Saitama, 2006
Similar to S. acuminatus
f. tetradesmiformis cell μm O-ike park Yabuki Fukushima, 2003

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