Family: Unicells or cells loosely attached forming hyphae; cell body varied in shape, a constriction at the center in most species; cell walls consisting of two or more parts, with small pores and various ornaments (Illustrations of The Japanese Fresh-water Algae, 1977).
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).

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Cell wall granulated or dotted
P. margaritaceum 28 samples 89-203 μm L/W= 4.2-8.0 1998-2008	P. margaritaceum 28 samples 89-203 μm L/W= 4.2-8.0 1998-2008	P. rufescens = Actinotaenium 5 samples 70 x 25-26 μm 2003-2008	P. cylindrus 10 samples 32-51 x 8-11 μm 2004-2008
Penium sp. ? μm Kuroiso Park Kuroiso Tochigi, 2003
Cell wall lined
P. polymorphum 35 samples 32-70 x 14-32 μm L/W=2.2-2.6 200-2008	P. polymorphum 35 samples 32-70 x 14-32 μm L/W=2.2-2.6 200-2008	P. polymorphum ? 35 x 13.5 μm Koigakubo marsh Tessei-cho Okayama, 2004
P. spirostriolatum 5 samples 210-240 x 20-22 μm L/W= 2006-2008	Penium sp. ? or Cylindrocystis ? 52 x 25 μm Hikage marsh Shiga highland Nagano, 2005	Penium sp. ? or Cosmarium ? 19 x 9 μm Koigakubo marsh Tessei-cho Okayama, 2004

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