Species [ var. globosum ]: Cell body small in size, slightly constricted at the center, 30-39 μm long, 20-28 μ wide, L/W=1.5, isthmus 19-26 μm wide; outline of semicells 2/3 of circle, apex rounded; cell wall dotted. [ var. wollei ]: Constriction stronger than that of var. globosum, 38.5-(42-43) μm long, 24-(33-39) μm wide, L/W < 1.5, isthmus 21-(31-37) μm wide, cell wall covered with small pores. [ var. subaltum ]: Longer than var. globosum, 31 μm long, 20 μm wide, isthmus 17 μm wide; cellwall dotted. [ var. minus ]: Smaller than var. globosum, 17-(21-22.5) μm long, 13-(15.4)-18 μm wide, isthmus 9.5-(9.8)-15 μm wide. [ f. minor ]: 16.8-27.4 μm long, 10.6-16.7 μm wide (Illustrations of The Japanese Fresh-water Algae, 1977).

Similar species -- >> C. diplosporum;

31 μm 63 μm 94 μm; x 640

Cosmarium globosum, cell body 33 μm long, 22 μm wide, L/W=1.5, x 640, Tanohara marsh, Shiga highland, Yamanouchi town, Nagano Pref., Japan, September 26, 2004 by Y. Tsukii
31 μm 63 μm 94 μm; x 640

Cosmarium globosum Bulnheim globosum, cell body 30 μm long, 15 μm wide, L/W=2.0, isthmus 13 μm wide, x 640, Tanohara marsh, Shiga highland, Yamanouchi town, Nagano Pref., Japan, September 26, 2004 by Y. Tsukii
31 μm 63 μm 94 μm; x 640