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 [ var. obsoletum]: Cell body medium in size, broad elliptical (wider than length), 34-(39-50) μm long, 42-(48-53)-54 μm wide, isthmus 15.5-(21-25) μm wide; semicells semielliptical or depressed semicircular, cell wall at basal corner thickened like a papillae; cell wall punctated and dotted.
[ var. sitvense ]: Cell body relatively large in size, 57-(69-78) μm long, 67-(69-78) μm wide, isthmus 30-(39-42) μm wide (Illustrations of The Japanese Fresh-water Algae, 1977).

Cosmarium obsoletum var. sitvense Gutwinski, tape No. 0067, time code 0849, Japan, Sep 29, 1998 by Y. Tsukii