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 without perforations, 12-74 μm in diam.; all cells with a deep incision, cell wall smooth ; marginal cells with two triangular processes extending outward, triangular (colonies with small number of cells) or trapezoid (large colonies) at base, 5-18 μm in diam., 4-18 μm long; inner cells five to seven angled, 4-16 μm in diam., 4-14 μm long (Photomicrographs of the Freshwater Algae, vol. 5, 1986).

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v. tetras 16 = 11 + 5 N/L=0.50 Mizumoto park Tokyo, 2001	v. tetras 8 = 7 + 1 N/L=0.40 Japan, 1998	v. tetras 8 = 7 + 1 N/L=4.5 Mashiko Tochigi, 1999	v. tetras 8 = 7 + 1 Ohnuma Nasu-shiobara Tochigi, 2007
v. tetras 16 = 11 + 5 Nashi-ike Kagami-ishi Fukushima, 2003	v. tetras 16 = 11 + 5 8 = 7 + 1 O-ike park Yabuki Fukushima, 2003	v. tetras 16 = 11 + 5 N/L=0.45 Uruguay, 1999	P. tetras or P. obtusum ? 16-, 8-, 4-cells 16 = 11 + 5 N/L= Uruguay, 1999
v. tetras 8 = 7 + 1 N/L= Sayama Natural p. Tokyo, 2001	v. tetras 8 = 7 + 1 Yakebe-ike Sayama Natural p. Tokyo, 2006
v. tetras 16 = 11 + 5 8 = 7 + 1 N/L= Nasu Tochigi, 2001	v. tetras 16 = 11 + 5 N/L= Mizumoto Park Tokyo, 2001	v. tetras 16 = 11 + 5 N/L=0.50 Japan, 1998 Movies	v. tetras 8 = 7 + 1 N/L=0.44 Shikinawa-ike Fukuoka, 2001
P. tetras 8 = 7 + 1 Higashi-hiroshima Hiroshima, 1998	v. tetras 8 = 7 + 1 Tane-ike Togakushi highland Nagano, 2005	v. tetras 8 = 7 + 1 Hasu-ike Shiga highland Nagano, 2005	v. tetras 8 = 7 + 1 Futayo-no-ike Haguro mountain Yamagata, 2005
v. tetraodon 8 = 7 + 1 N/L=0.56 Saitama, 1999	P. tetras var. tetraodon 16 = 11 + 5 N/L=0.55 Tsuchiura Ibaraki, 2002	v. tetraodon ? 8 = 7 + 1 N/L=0.56 Wan-park-land Kochi, 2002	v. tetraodon ? 16 = 11 + 5 N/L=0.63 Tatebayashi Gunma, 2001
P. tetras 16 = 11 + 5 Kurome river Asaka Saitama, 1998	v. tetraodon ? 8 = 7 + 1 N/L= Chichibu Saitama, 2003	v. tetraodon ? 8 = 7 + 1 N/L= Aobano-mori Park Chiba, 2003	v. tetraodon ? N/L= Ranzan Saitama, 2004
P. tetras var. tetraodon ? 8 = 7 + 1 N/L= Chiba-Minato Park Chiba, 2003	P. tetras N/L= Zenko-ji Nagano, 2003	P. tetras N/L= Naruto Chiba, 2003	P. tetras N/L= near City Hall Fujimi Saitama, 2004
P. tetras N/L= Higashi-sano Hakuba Nagano, 2004	P. tetras N/L= Nozomiko 2 Iiyama Nagano, 2007
v. tetraodon N/L= water reservoir near Yamaguchi Univ. Yamaguchi, 2004	v. tetraodon N/L= Aizu Recreation p. Aizuwakamatsu Fukushima, 2005
Colony of 4 cells
v. tetras or v. tetraodon ? colonies of 4 cells Kabuto-yama Nishi-no-miya Hyogo, 2004	v. tetras ? colonies of 4 cells water reservoir near Yamaguchi Univ. Yamaguchi, 2004	v. tetras ? N/L= Higusa-numa Hikari Chiba, 2003	v. tetras ? N/L= Tane-ike Togakushi highland Nagano, 2005
v. tetraodon colonies of 4 cells Ushijima park Kasukabe Saitama, 2003	v. tetraodon ? stock Sksu-1 colonies of 4 cells Ushijima park Kasukabe Saitama, 2003	v. tetraodon ? μm near Yamaguchi Univ. Yamaguchi, 2004	v. tetraodon ? 17 μm Nagaike park Hachioji Tokyo, 2005
v. tetraodon ? Higashi-hiroshima Hiroshima, 1998	P. tetras ? N/L= Tsugaike highland Otari Nagano, 2006

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