Genus: Cell body short, radiate in shape; apical view of the cell, triangular, polygonal, ellipsoidal or spindle-shaped; zygospores formed outside of mother cells (Illustrations of The Japanese Fresh-water Algae, 1977)
Subgenus: lacks arm-like projections (Illustrations of The Japanese Fresh-water Algae, 1977)
Species [var. avicula ]: Cell body 21-26 μm long, 22.4-26 μm wide, isthmus 7-8.6 μm wide
[var. subarcuatum (Wolle) W. et G.S. West ]: Cell body 21-31 μm long, 19.6-31 μm wide, isthmus 7-11 μm wide (Illustrations of The Japanese Fresh-water Algae, 1977)

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S. avicula v. subarcuatum 24 x 30 μm Numappara marsh Kuroiso Tochigi, 2003	S. avicula v. subarcuatum ? 24 x 26 μm Higashi-sano Hakuba Nagano, 2004	S. avicula v. subarcuatum 24 x 26 μm Karakemi marsh Omachi Nagano, 2006	S. avicula v. subarcuatum 25 x 25 μm Dojo-ike Norikura highland Nagano, 2006
S. avicula v. subarcuatum μm Dojo-ike Norikura highland Nagano, 2007	S. avicula v. subarcuatum 21 x 21 μm Maimeno-ike Norikura highland Nagano, 2006	S. avicula v. subarcuatum 24 x 28 μm Kashibaru marsh 5 Karatsu Saga, 2006	S. avicula v. subarcuatum 27 x 27 μm Nozomiko 2 Myoko-kogen Nagano, 2007

Similar species -- >> S. subavicula;

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