Section [Margaritiferae]:
Cell surface granulated, gnarled or spiny; semicells pyramidal with flattened apex or trapezoid
(Illustrations of The Japanese Fresh-water Algae, 1977).
Species [var. subcostatum]:
Cell body 26-38 μm long, 23-32 μm wide, isthmus 6.5-12 μm.
[f. minor W. et G.S. West]:
Cell body 19-26 μm long, 18-23 μm wide, isthmus 4.5-9 μm.
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