Species [var. contractum]: Cell body 28-41 μm long, 18-31 μm wide, isthmus 5.3-8.7 μm wide, L/W=1.5; semicells broad ellipsoid, laterally circular; cellwall smooth or finely dotted, a single pyrenoid in each semicell. [var. ellipsoideum (Elfving) W. et G.S. West]: Cell body 31-51 μm long, 24-42 μm wide, isthmus 7-12.5 μm wide, L/W=1.3; apex of semicells slightly flattened; cellwall smooth. [f. jacobsenii (Roy) W. et G.S. West]: Semicells exactly ellipsoid, 28-47.6 μm long, 18-36.4 μm wide, isthmus 4.5-11.3 μm wide. [var. minutum (Delponte) W. et G.S. West]: Cell body 19-28 μm long, 14-22 μm wide, isthmus 4.3-5.6 μm wide; semicells elongated ellipsoid, laterally circular, cellwall dotted. (Illustrations of The Japanese Fresh-water Algae, 1977).

Similar species -- >> C. depressum; C. bioculatum; C. moniliforme;

31 μm 63 μm 94 μm; x 640

Cosmarium contractum Kirchner var. contractum, axenic culture (from December 19, 2007 to February 1, 2008), cell body μm long, μm wide, L/W=, x 640, No.1, Kashibaru marsh, Karatsu city, Saga Pref., Japan, November 2006 by Y. Tsukii
31 μm 63 μm 94 μm; x 640

Images of collecting locality: No.1, Kashibaru marsh, Karatsu city, Saga Pref., Japan, November 22, 2006 by Y. Tsukii