Genus: Colonies of 4-32 cells; cell body long crescent or spindle in shape, tapered at both ends; cells attached with each other at the center of the body, paralleled or radially arranged; no gelatinous sheath; a chloroplast; no pyrenoids; planktonic (Guide book to the "Photomicrographs of the Freshwater Algae", 1998).
Species: Cell body needle-shaped, tapered to the sharply pointed ends, slightly twisted or curved, 25-100 μm long, 1.5-6 μm wide; a single chloroplast parietal, plate-like, usually without pyrenoids (Photomicrographs of the Freshwater Algae, 1985). 25-30 μm long, 2-3 μm wide (Illustrations of The Japanese Fresh-water Algae, 1977).

Cell body relatively thick
A. falcatus 50 x 3 μm attached at center body straight Nishi-nasuno Tochigi, 2001	A. falcatus 55 x 3 μm attached at center body straight Nasu Tochigi, 2001	A. falcatus μm attached at center body straight Kawashima or Kawajima Saitama, 2002	A. falcatus 35 x 3 μm attached at center Kejo-numa Furukawa Miyagi, 2003
A. falcatus 61 x 3 μm loose colony b. slightly curved Nishi-nasuno Tochigi, 2001	A. falcatus 63 x 2.5 μm loose colony Kannon-numa Shimogo Fukushima, 2005	A. falcatus μm loose colony Kannon-numa Shimogo Fukushima, 2008	A. falcatus 51 x 3 μm loose colony b. slightly curved Urawa Saitama, 2001
A. falcatus ? 62 x 3 μm attached at center b. slightly curved Saitama, 1999	A. falcatus x 2.5 μm cells stacked b. slightly curved Showa Memorial P. Tokyo, 2001	A. falcatus 65 x 2.5 μm cells stacked Murakuni ike Iwatsuki-bunka p. Saitama, 2006
Cell body relatively thin
A. falcatus 50 x 2 μm loose colony b. slightly curved Nishi-nasuno Tochigi, 2001	A. falcatus ? 62 x 1.5-2 μm cells stacked body wavy Saitama, 1999	A. falcatus 47 x 2 μm loose colony b. slightly curved Sanda Hyogo, 2001	A. falcatus μm loose colony b. slightly curved Chiba-Minato Park Chiba, 2003
A. falcatus μm loose colony Omine-numa Tsukiyono Gunma, 2003	A. falcatus 62 x 2 μm loose colony Bessho-numa Saitama, 2001	A. falcatus 38 x 2 μm loose colony b. slightly curved Japan, 1998	A. falcatus ? 63 x 2 μm loose colony Japan, 1998
A. falcatus ? μm loose colony Takamatsu Kagawa, 2002	A. falcatus ? μm Omine-numa Tsukiyono Gunma, 2003	A. falcatus 40-50 μm Iruma river Kawagoe Saitama, 2004	A. falcatus 60-65 x 1-1.5 μm Lake Shirakaba Tateshina & Chino Nagano, 2005
A. falcatus 52,61 x 2,2.5 μm Kannon-numa Shimogo Fukushima, 2005	A. falcatus 70 x 2 μm Maimeno-ike Norikura highland Nagano, 2006

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