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).

31 μm 63 μm 94 μm; x 640

Cosmarium obsoletum var. sitvense (left) and C. obsoletum var. obsoletum (right), cell body μm long, μm wide, x 640, Omine-numa (pond/marsh), Tsukiyono town, Gunma Pref., Japan, September 2003 by Y. Tsukii
31 μm 63 μm 94 μm; x 640