Species [var. costatum]: Cell body medium in size, inner side straight, outer side strongly convex, both termini conical, 200-(304-385)-550 μm long, (28-41)-70 μm wide, L/W=6-8; cell wall brownish in color, striae separated each other (5-6 within 10 μm), ridged at both termini; chloroplasts with 5-6 laminae, 6-8 pyrenoids in each semicell.
[var. westii]: Cell body more elongate, curved, 230-(336-480) μm long, 23-(30-43) μm wide, L/W=8-11; striae thin at the center, thick at both termini; small dots present between the striae (Illustrations of The Japanese Fresh-water Algae, 1977).

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C. costatum v. costatum ? 190 x 27 μm Vermont-Maine U.S.A., 1999	C. costatum v. westii 364-460 x 40-41 μm Imori pond Myoko highland Niigata, 2004	C. costatum v. westii 343 x 39 μm near Imori pond Myoko highland Niigata, 2004	C. costatum v. westii 415 x 36 μm Ohnuma Nasu-shiobara Tochigi, 2006
C. costatum v. westii 440,660 x 52,63 μm Karakemi marsh Omachi Nagano, 2006	C. costatum v. westii 276,430 x 39,38 μm Kodanuki marsh Fujinomiya Shizuoka, 2006	C. costatum v. westii 500 x 37 μm Kashibaru marsh 4 Karatsu Saga, 2006	C. costatum v. westii 343 x 41 μm Kareki-numa m Nikko Tochigi, 2008

Similar species -- >> C. striolatum; C. intermedium; C. angustatum;

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