Genus: Unicellular; variable in shape; a constriction at the center of the cell body; mostly longer than wide; flattened; each semicell hemispherical, spherical, ellipsoidal, rectangular, pyramidal or kidney-shaped; no apical indentation
Subgenus [Eucosmarium]: Cell body dorso-ventrally compressed, with strong constriction at the center, transverse section elliptical or diamond-shaped.
Section [Microcosmarium]: Cell surface smooth or dotted or punctated; semicells polygonal (Illustrations of The Japanese Fresh-water Algae, 1977).
Species: Cell body small, (21-29.5)-36 μm long, (15.4-18)-26 μm wide, isthmus 5-(5.6-7)-9 μm wide, L/W=1.5; both sides of semicells with four wavy margins, all waves equal in size, side view elongated ellipsoid; cell wall smooth (Illustrations of The Japanese Fresh-water Algae, 1977).

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C. impressulum var. impressulum 32 x 23 μm Ishinazaka Furukawa Miyagi, 2003	C. impressulum var. impressulum 29 x 19 μm Saitama, 1999	C. impressulum var. impressulum 25 x 19 μm Koshigaya Saitama, 2001	C. impressulum 25 x 17 μm Konuma Sakado Saitama, 2004
C. impressulum 25 x 17 μm rice field Ichinoseki Iwate, 2005	C. impressulum var. impressulum 23 x 18 μm Toneri Park Tokyo, 2001	C. impressulum 22 x 14 μm Norin Park Kawamoto Saitama, 2004	C. impressulum 22 x 19 μm Univ. A.V.M. Obihiro Hokkaido, 2005
C. impressulum var. ?? or C. scopulorum ? 21 x 13 μm Kurihashi Saitama, 2001	C. impressulum 19 x 14 μm Muneoka Shiki Saitama, 2004	C. impressulum 18.5 x 14 μm Furuyakami Kawagoe Saitama, 2004	C. impressulum 21 x 15 μm Shimo-muneoka Shiki Saitama, 2004

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