Species [var. subtumidum]: Cell body (28-39)-40 μm long, 21-33.6 μm wide, isthmus 7-(8.5-14) μm wide. [var. rotundum Hirano ]: Cell body 29-30 μm long, 21 μm wide, isthmus 7 μm wide. [var. klebsii (Gutwinski) W. et G.S. West ]: Cell body 31-47 μm long, 31-43 μm wide, isthmus 8.5-12 μm wide. [f. minor Ström ]: Cell body 11-25 μm long, 11-20 μm wide, isthmus 4.2-5.6 μm wide (Illustrations of The Japanese Fresh-water Algae, 1977).

50 μm 100 μm 150 μm; x 400 : 31 μm 63 μm 94 μm; x 640

Cosmarium circulare var. minor (or Cosmarium subtumidum var. klebsii ?), cell body 45 μm long, 37 μm wide, constriction 16 μm wide, x 400, x 640, Saitama Pref., Japan, May-June 1999 by Y. Tsukii

Cosmarium circulare var. minor (or Cosmarium subtumidum var. klebsii ?), cell body 41 μm long, 37 μm wide, constriction 16 μm wide, x 400, x 640, Saitama Pref., Japan, May-June 1999 by Y. Tsukii