Class Conjugatophyceae Engl. (1892) see Class Zygnematophyceae Round ex Guiry (2013)
   Order Zygnematales - filamentous conjugates and saccoderm desmids
     Family Zygnemataceae (Meneghini) Kützing (1843)
       Genus Trigonum A.K. Mahato & P. Mahato (1994) [monotypic]
         1 Trigonum indicum A.K. Mahato & P. Mahato