Genus: Cell body cylindroid, both ends rounded; cell wall smooth or granulated or longitudinally lined; constriction or bands present in some species; outer cell wall yellowish or brownish; chloroplast has radially-arranged laminae which attached cell wall in full length; one or several pyrenoids in each semicell, aligned in chloroplasts (Illustrations of The Japanese Fresh-water Algae, 1977).
Species [ var. cylindrus ]: Cell body cylindrical, both ends truncated with rounded corners, (48-56)-58 μm long, 8-(11-13)-21 μm wide, L/W=5.0; cell wall dotted, brownish in color when aged, bands present.
[ var. cuticulare ]: Smaller than var. cylindrus, 19-34 μm long, 8-10 μm wide, L/W=3; (Illustrations of The Japanese Fresh-water Algae, 1977).

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Cell wall granulated or dotted
P. cylindrus 50 x 15 μm L/W=3.3 Shijuhachi-ike 2 Shiga highland Nagano, 2006	P. cylindrus v. cylindrus ? 42 x 11 μm L/W=3.8 Iroha-numa Zao mountain Yamagata, 2005	P. cylindrus v. cylindrus ? 40-67 x 11 μm L/W=3.6-6.1 Iroha-numa 2 Zao mountain Yamagata, 2006	P. cylindrus v. cylindrus ? 35-39 x 11 μm L/W=3.2-3.4 Iroha-numa 3 Zao mountain Yamagata, 2006
P. cylindrus v. cuticulare 32 x 9 μm L/W=3.6 Imou bog Toyohashi Aichi, 2004	P. cylindrus v. cuticulare 45 x 11.5 μm L/W=3.9 Kashibaru m. Karatsu Saga, 2008	P. cylindrus v. cuticulare 41-51 x 11.5 μm L/W=3.9-4.4 Kareki-numa m. Nikko Tochigi, 2008.8	P. cylindrus v. cuticulare 45 x 13 μm L/W=3.5 Kareki-numa m. Nikko Tochigi, 2008.9
P. cylindrus v. cuticulare ? 68 x 8.5 μm L/W=8.0 Kareki-numa m. Nikko Tochigi, 2008.8	P. cylindrus v. cuticulare ? 75 x 8 μm L/W=9.4 Kareki-numa m. Nikko Tochigi, 2008.9

Similar species -->> P. exguum;

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