Genus: Cell body small, length = width, strong constriction at its center; semicells inverted triangular, ellipsoidal, rectangular, etc. in shape; straight or slightly curved spiny projections at each corner; one pyrenoid in each semicell; cell wall smooth (Illustrations of The Japanese Fresh-water Algae, 1977).
Species:

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var. latiusculum:
A. triangularis var. latiusculum 22 x 20 μm near Kakuman-buchi Akagi mt. Gunma, 2007	A. triangularis var. latiusculum μm Kareki-numa Nikko Tochigi, 2008	A. triangularis var. latiusculum 22 x 22 μm Kowashimizu Suwa Nagano, 2007	A. triangularis ? var. latiusculum ? 21 x 27 μm Tanohara marsh Shiga highland Nagano, 2004
A. triangularis ? or A. incus ? 17 x 15 μm Shimo-no-koike Shiga highland Nagano, 2006
var. undulatus:
A. triangularis v. undulatus 23 x 42 μm Uruguay, 1999

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