Species: [ var. parvulum ]: Cell body curved, not swelled at the center (inner side straight or slightly concaved), tapered toward both thorn-like or slightly rounded ends (cf. C. calosporum), a small pore present on its outer side, 60-160 μm long, (8.5-15)-17 μm wide, L/W=7-10; cell wall smooth, transparent; chloroplasts with fewer laminae (only two visible); 3-5 pyrenoids in each chloroplast. [ var. angustum ]: Elongate and thinner than var. parvulum, 75-(98-141) μm long, 5-(8.5-11) μm wide, L/W=11-15; curvature variable (Illustrations of The Japanese Fresh-water Algae, 1977).

31 μm 63 μm 94 μm; x 640

Images of collecting locality: No.5, Oh-numa marsh, Akita-Hachimantai, Kazuno city, Akita Pref., Japan, September 21, 2007 by Y. Tsukii