Genus: Variable in shape; strong constriction at center; semicell ellipsoidal, polygonal, etc.; cell wall with simple or branching spines, most of them laterally aligned; 4 plate-like chloroplasts in each semicell; one or more pyrenoids in each chloroplast (Illustrations of The Japanese Fresh-water Algae, 1977).
Species:

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X. armatum 127 x 80 μm Kenashitai Hakkoda mountain Aomori, 2005	X. armatum 127 x 97 μm Shimo-Kenashitai 1 Hakkoda mountain Aomori, 2006	X. armatum 97 μm Ohyachi m.2 Hachimantai Akita, 2007
X. armatum μm Kuroyachi m.2 Hachimantai Iwate, 2007	X. armatum μm Mitsuishi m.4 Hachimantai Iwate, 2008
X. armatum 122 x 83 μm Iroha-numa Zao mountain Yamagata, 2005	X. armatum 135 x 85 μm Iroha-numa Zao mountain Yamagata, 2006	X. armatum μm Iroha-numa Zao mountain Yamagata, 2008	X. armatum 115 x 90 μm Iroha-numa Azuma mt. Yamagata, 2007
X. armatum μm Miyatoko m.3 Minami-Aizu Fukushima, 2008	X. armatum 115 x 75 μm Oze marsh Katashina Gunma, 2006	X. armatum 125 x 90 μm Shijuhachi-ike 4 Shiga highland Nagano, 2007	X. armatum μm Shijuhachi-ike 1 Shiga highland Nagano, 2008
X. armatum 125 x 85 μm Shimo-no-koike Shiga highland Nagano, 2004	X. armatum 113 x 80 μm Shimo-no-koike Shiga highland Nagano, 2005	X. armatum 115 x 80 μm Shimo-no-koike Shiga highland Nagano, 2006	X. armatum 125 x 85 μm Shimo-no-koike Shiga highland Nagano, 2007
X. armatum 115 x 90 μm Kamino-tanbo Tsugaike highland Nagano, 2006	X. armatum 120 x 80 μm Maimeno-ike Norikura highland Nagano, 2006	X. armatum 120 x 77 μm Ushidome-ike Norikura highland Nagano, 2005	X. armatum 116 x 80 μm Ushidome-ike Norikura highland Nagano, 2006
X. armatum ? 69 x 56 μm Watarase Tochigi, 2001

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