Species: Cell body 58-75 μm long, 26.5-31 μm wide, L/W=2-2.5, isthmus 20-29 μm wide; semicells broad ellipsoidal or ovoidal, apex broad circular or narrowly flattened; cell wall finely dotted; band-form chloroplasts aligned along inner cellwall (Illustrations of The Japanese Fresh-water Algae, 1977).

Similar species -->> C. oblongum; C. cucurbita; C. cucurbitinum; C. diplosporum; C. globosum;

31 μm 63 μm 94 μm; x 640

Cosmarium thwaitesii Ralfs ??, cell body μm long, μm wide, L/W=, isthmus μm wide, x 640, No.2, Hachiman-numa marsh, Hachimantai, Hachimantai city (formerly Ashiro town & Matsuo village), Iwate Pref., Japan, September 12, 2006 by Y. Tsukii
31 μm 63 μm 94 μm; x 640

Images of collecting locality: No.2, Hachiman-numa marsh, Hachimantai, Hachimantai city (formerly Ashiro town & Matsuo village), Iwate Pref., Japan, September 12, 2006 by Y. Tsukii