Genus: Cell body crescent in shape (long, curved, tapered at both ends); cross section rounded; cell wall smooth or with longitudinal lines, bands on the cell wall in some species; chloroplast laminae ridged and radially arranged, which makes cross section star-shaped; pyrenoids aligned or scattered (Illustrations of The Japanese Fresh-water Algae, 1977). Species [var. variabile]: Cell body strongly curved, nearly semicircular, 43-140-(150) μm long, 2.5-5-(6) μm wide (Illustrations of The Japanese Fresh-water Algae, 1977).

31 μm 63 μm 94 μm; x 640

cf. C. toxon W. West: 163-(190-350) μm long, 8-(8.4-13)-20 μm wide; L/W = 25-33; cell body slightly curved; both termini broadly rounded (nearly truncated); cell wall transparent, smooth; 5-7 pyrenoids arranged in a single line (Illustrations of The Japanese Fresh-water Algae, 1977).

C. cornu Ehrenberg var. cornu: Cell body slightly curved, tapered at rear, L/W= 16-17; both termini broad circle; cell wall smooth, transparent; chloroplasts with 3-4 pyrenoids in each semi-cells; 95-160 μm long, 5-(7.8-13) μm wide (Illustrations of The Japanese Fresh-water Algae, 1977).