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. acuminatus ]: Colonies of 4 or 8 cells; cells arranging linearly (4 cells) or zigzag (8 cells); cell body elongated spindle-shaped, outer cells curved outward, 12-30 μm long, 2.5-6 μm wide; cell wall smooth (Illustrations of The Japanese Fresh-water Algae, 1977).

31 μm 63 μm 94 μm; x 640

S. acuminatus (Lagerheim) Chodat var. acuminatus: Colonies of 4 or 8 cells; in the 4-cell type, cells attached at center; in the 8-cell type, cells attached in a zigzag pattern; cell body long spindle in shape; cells at both end curved; cell wall smooth; 12-30 μm long, 2.5-6 μm diam. (Illustrations of The Japanese Fresh-water Algae, 1977).

S. acuminatus f. maximus Uherkovich: Larger than var. acuminatus; 28-50 μm long, 7-9 μm diam. (Illustrations of The Japanese Fresh-water Algae, 1977).

S. acuminatus f. tortuosus (skuja) Uherkovich: Cells at both end strongly curved; 20-45 μm long, 3-7 μm diam. (Illustrations of The Japanese Fresh-water Algae, 1977).

S. acuminatus var. elongatus G. M. Smith: Only two cells attached at center; both cells curved outside; 40-50 μm long, 2-5 μm diam. (Illustrations of The Japanese Fresh-water Algae, 1977).