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 Peniaceae Haeckel (1894) [paraphyletic]
         Genus Penium Bréb. ex Ralfs (1848)
           1 Penium margaritaceum (Ehrenb.) ex Ralfs (1848)
           2 Penium spirostriolatum Barker
           3 Penium exiguum West (1892)