Genus: Trichome without mucous sheath; straight, long, consisting of numerous cells (Illustrations of The Japanese Fresh-water Algae, 1977).
Species: Trichome dark-blue in color, nearly straight, without constriction at cell-cell junction, 6-11 μm wide; cell body reddish blue in color, 4-11 μm long (1/4-1/2 of its width); granules present along septum (cf. granules absent -> O. simplissima); cell wall of the terminal cells thick (Illustrations of The Japanese Fresh-water Algae, 1977).

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O. irrigua ? or O. curviceps ? 11 μm φ L/W= Ta river Ishibashi Tochigi, 2004	O. irrigua ? 9 μm φ L/W= Omoi river Oyama Tochigi, 2004	O. irrigua ? 3.5-4.5 x 9 μm φ L/W=0.4-0.5 near Yamaguchi Univ. Yamaguchi, 2004	O. irrigua ? 9 μm φ L/W= Mizuko Fujimi Saitama, 2003
O. irrigua ? 8.5 μm φ L/W=0.6-1.2 Akagi forest p. Fujimi Gunma, 2005	O. irrigua ? (left) 8 μm φ L/W=0.4-0.8 Tanohara Natural p. Ontake highland Nagano, 2005	O. irrigua ? (left) 4-8 μm φ L/W= Imou marsh Toyohashi Aichi, 2006	O. irrigua 9 μm φ L/W= Kashibaru marsh Karatsu Saga, 2006
O. irrigua ? 6 μm φ L/W= Kurome river Asaka Saitama, 2004	O. irrigua ? 2-3 x 7 μm L/W=0.3-0.4 Akigase Park Saitama, 2002	O. irrigua ? 7.5 μm φ L/W= Nasuno-ga-hara m. Nishi-nasuno Tochigi, 2004	O. irrigua ? 5-7 x 7.5 μm L/W=0.7-0.9 Shimo-no-koike Shiga highland Nagano, 2004
O. irrigua ? 3-6 x 5 μm L/W=0.6-1.2 Kitamoto nature o.p. Kitamoto Saitama, 2005
O. irrigua ? or O. simplicissima ? 9.5 μm φ L/W= O-ike park Yabuki Fukushima, 2003	O. irrigua ? or O. simplicissima ? 9.5 μm φ L/W= Furu-tone river Kasukabe Saitama, 2004	O. irrigua ? or O. simplicissima ? 8.5 μm φ L/W= Motono Chiba, 2003	O. irrigua ? or O. simplicissima ? 2-4 x 9 μm L/W= water treatment c. Sakado Saitama, 2004

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