Genus: Colonies of a fixed number of cells, flat, circular in shape; cell body polygonal in shape, with horn-like projections; cell wall garanulated, wrinkled or notched; chloroplasts plate-like or reticular; asexual reproduction by zoospore; sexual reproduction by isogametes (Illustrations of The Japanese Fresh-water Algae, 1977).
Species: Colonies of 8-32 (rarely 4, 64 or 128 cells) with large intercellular space; inner cells H or X-shaped, trapezoid to rectangular when matured; outer half of the marginal cells prolonged into two horn-like processes with a tuft of long bristles; daughter colonies formed in a vesicle released from mother cell; cell wall smooth (Photomicrographs of the fresh-water algae, 1994).

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var. duplex - var. reticulatum
A cell located at the center of colonies
var. duplex 16-cells 16 = 10+5+1 Shakujii Park Tokyo, 2001	var. duplex 16-, 8-cells 16 = 10+5+1 Japan, 1997	var. duplex 32-, 16-cells 16 = 10+5+1 Tochigi, 2001	var. duplex 32-, 16-, 8-cells 16 = 10+5+1 Japan, 1998
var. duplex stock Smt-51 16 = 10+5+1 Minuma-tanbo Saitama, 2001	var. duplex Nasu Tochigi, 2001	var. duplex -cells Wan-park-land Kochi, 2002	var. duplex ? stock Tna-32 Nasu Tochigi, 2001
var. duplex ? 16 = 10+5+1 Tokyo, 2001	var. duplex ? 16 = 10+5+1 Daidarabotchi-ike Iizuna-kogen Nagano, 2003	var. duplex 32 = ??+1 Tama river Komae Tokyo, 2004	var. duplex 16-cells 16 = 10+5+1 near Town Hall Shobu Saitama, 2004
var. duplex ? colony 54 μm φ near Okubo ike Shinano Nagano, 2004	var. duplex 32-cells 32 = 15+10+6+1 Kejo-numa 1 Osaki Miyagi, 2006
var. reticulatum ? 16-cells Yamaguchi Univ. Yamaguchi, 2004	var. reticulatum 16-cells riverside park Abukuma river Fukushima, 2005	var. reticulatum 8-cells Showa-machi g.p. Showa Saitama, 2006

Similar species -->> P. angertiniense

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