Cell body dorso-ventrally compressed, with strong constriction at the center,
transverse section elliptical or diamond-shaped.
Section [ Cyclidium ]: Cell surface smooth; semicells half circle, half ellipsoid (Illustrations of The Japanese Fresh-water Algae, 1977).
Species[ magnum ]: Cell body 78-84 μm long, 78-80 μm wide, isthmus 39-44 μm wide; (Illustrations of The Japanese Fresh-water Algae, 1977).
Cosmarium taxichondrum Lundell ?,
cell body 90 μm long, 85 μm wide, L/W=1.1, isthmus 50 μm wide,
x 640, Koigakubo marsh, Tessei-cho, Okayama Pref., Japan, November 22, 2004 by Y. Tsukii
31 μm 63 μm 94 μm; x 640