Species [ var. formosulum ]: Cell body small, trapezoid in shape, basal margins broadly rounded, both sides crenated with 6-7 projections, apex slightly wavy (5 waves), (34)-40-50 μm long, (34)-40 μm wide, isthmus 10-(11)15.5 μm wide; cell wall granulated; semicells with about 5 longitudinal ridges at the center, granules present on each ridge, side view ovoid (Illustrations of The Japanese Fresh-water Algae, 1977). [var. mesochondrium]: Cell body 37.8-39 μm long, 30-31 μm wide, isthmus 9.8-11 μm wide, (Illustrations of The Japanese Fresh-water Algae, 1977).

31 μm 63 μm 94 μm; x 640

Cosmarium formosulum, cell body μm long, μm wide, constriction μm wide, x 400, x 640, Kurihashi, Saitama Pref., Japan, July 2001 by Y. Tsukii
31 μm 63 μm 94 μm; x 640

Cosmarium formosulum Hoffmann var. formosulum: Cell body (34)-40-50 μm long, (34)-40 μm wide, isthmus 10-(11)15.5 μm wide, (Illustrations of The Japanese Fresh-water Algae, 1977).

Cosmarium formosulum var. mesochondrium (Schmidle) Hirano: Cell body 37.8-39 μm long, 30-31 μm wide, isthmus 9.8-11 μm wide, (Illustrations of The Japanese Fresh-water Algae, 1977).