Genus: Highly flattened; pellicle firm; body form constant (Kudo, 1966). Chloroplasts small, discoid, pyrenoids usually absent; most species flat and leaf-shaped; often with ridges or fins running helically or longitudinally (Illustrated Guide, 1985).
Species:

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P. obolus 55 x μm Mihashi Park Saitama, 2003	P. obolus 38 x 22 μm Motono Chiba, 2003	P. obolus μm Mizuko-kaizuka p. Fujimi Saitama, 2004	P. obolus 45 x 24 μm water reservoir near Yamaguchi Univ. Yamaguchi, 2004
P. obolus 37 x 22 μm Shiba river near Owada Park Saitama, 2005	P. obolus 43 x 23 μm Ichinose-enchi Norikura highland Nagano, 2005	P. obolus 45 x 23.5 μm Kitamoto n.o.p. Kitamoto Saitama, 2006

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