Freshwater Algae of North America Class Zygnematophyceae C.Hoek, D.G.Mann & H.M. Jahns (1995), nom. inval. - conjugating green algae see Class Zygnematophyceae Round ex Guiry (2013) Order Desmidiales - placoderm desmids Family Desmidiaceae Kützing ex Ralfs (1845) Genus Xanthidium Ehrenb. ex Ralfs (1848) Xanthidium armatum (Bréb.) ex Ralfs (1848)