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: Cell body cylindrical or ellipsoidal, tapered to both ends, 35-79 μm long, (16-21)-36 μm wide; both ends broadly rounded; cell wall thin, brownish in color, finely lined (15-18/10 μ) (Illustrations of The Japanese Fresh-water Algae, 1977).

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Cell body wider than 20 μm
P. polymorphum 62-70 x 28-32 μm Shimo-Kenashitai 5 Hakkoda mt. Aomori, 2006	P. polymorphum 55 x 24 μm Sukawa highland Kurikoma mountain Akita, 2005	P. polymorphum 60 x 23 μm Sukawa highland Kurikoma mountain Akita, 2007	P. polymorphum 61 x 25 μm L/W=2.4 Iroha-numa Zao mountain Yamagata, 2005
P. polymorphum 63 x 25 μm L/W=2.5 Iroha-numa Zao mountain Yamagata, 2008	P. polymorphum 65 x 24 μm Komado marsh Showa & Tajima Fukushima, 2004	P. polymorphum 56 x 23 μm Shirakaba-Yachi 1 Komado marsh Fukushima, 2007	P. polymorphum ? 70 x 26 μm Mizunashi-Yachi 2 Komado marsh Fukushima, 2007
P. polymorphum 66 x 25 μm Yokotashiro 1 Oze marsh Gunma, 2007	P. polymorphum 57 x 27 μm Yokotashiro 4 Oze marsh Gunma, 2008
P. polymorphum or Netrium ? 54-70 x 18-20 μm Numa-no-hara m. Madarao highland Niigata, 2005	P. polymorphum 58 x 24 μm Midagahara 6 Tateyama Toyama, 2006	P. polymorphum 52 x 23 μm Hachiman-numa Hachimantai Iwate, 2006
P. polymorphum 54 x 23.5 μm 75 x 24 μm Tanohara marsh Shiga highland Nagano, 2004	P. polymorphum 58 x 24 μm Tanohara marsh Shiga highland Nagano, 2006	P. polymorphum 55 x 23 μm Tanohara marsh 2 Shiga highland Nagano, 2007
P. polymorphum 63 x 23 μm Shibu-ike Shiga highland Nagano, 2005	P. polymorphum 52-70 x 25-32 μm Shijuhachi-ike Shiga highland Nagano, 2006	P. polymorphum 52 x 23 μm L/W=2.3 Hikage marsh Shiga highland Nagano, 2005	P. polymorphum 52,55 x 23.5 μm L/W=2.3 Hikage marsh Shiga highland Nagano, 2006
P. polymorphum 53 x 23 μm L/W= Chigo-ike m.2 Shiga highland Nagano, 2006	P. polymorphum 56-64 x 21-25 μm L/W=3.0 Chigo-ike m.4 Shiga highland Nagano, 2006	P. polymorphum 59 x 22 μm L/W= Chigo-ike Shiga highland Nagano, 2006	P. polymorphum 53 x 25 μm L/W=2.1 Chigo-ike 2 Shiga highland Nagano, 2008
P. polymorphum 56,58 x 28 μm L/W=2.0-2.1 Chigo-ike 3 Shiga highland Nagano, 2008
P. polymorphum 60 x 24 μm L/W= Maeyama marsh Shiga highland Nagano, 2007	P. polymorphum 47 x 20 μm Tenbo marsh Tsugaike highland Nagano, 2005	P. polymorphum 46,57 x 23 μm J Skunk Cabbage m. Tsugaike highland Nagano, 2006	P. polymorphum 49-62 x 20-22 μm Watasuge marsh Tsugaike highland Nagano, 2006
P. polymorphum 63 x 25 μm Ichinose-enchi Norikura highland Nagano, 2006	P. polymorphum 55-66 x 25 μm L/W=2.2-2.6 Tanohara Natural p. Ontake highland Nagano, 2005
Cell body narrower than 20 μm
P. polymorphum 39 x 17 μm Shimo-Kenashitai 1 Hakkoda mt. Aomori, 2006	P. polymorphum 40 x 17 μm Miyatoko m.3 Minami-Aizu Fukushima, 2008	P. polymorphum ? 39 x 16 μm Koigakubo marsh Tessei-cho Okayama, 2004	P. polymorphum ? 31 x 15 μm Watasuge marsh Tsugaike highland Nagano, 2005
P. polymorphum 35 x 13.5 μm Koigakubo marsh Tessei-cho Okayama, 2004

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