Genus: Unicellular; variable in shape; a constriction at the center of the cell body; mostly longer than wide; flattened; each semicell hemispherical, spherical, ellipsoidal, rectangular, pyramidal or kidney-shaped; no apical indentation (Illustrations of The Japanese Fresh-water Algae, 1977). Species: Cell body small, (14-18)-22 μm long, (11-16.8)-22 μm wide, isthmus (11-16.8)-22 μm wide; (Illustrations of the Japanese fresh-water algae, 1977).

31 μm 63 μm 94 μm; x 640

Cosmarium regnellii (or C. abbreviatum ?), cell body 14 μm long, 14 μm wide, isthmus 5 μm wide, x 640, Norin Park (Agricultural and Forest park), Kawamoto town, Saitama Pref., Japan, August 16, 2004 by Y. Tsukii
31 μm 63 μm 94 μm; x 640

Cosmarium regnellii Will var. regnellii: Cell body small, (14-18)-22 μm long, (11-16.8)-22 μm wide, isthmus (11-16.8)-22 μm wide; (Illustrations of the Japanese fresh-water algae, 1977).