Phylum: Charophyta   Family: Desmidiaceae

Cosmarium regnesi var. tritum West 1892

The main difference from the nominate variety is a 'V-shaped' sinus, giving a more triangular rather than rectangular shape. It is also a little smaller. It needs to be closely compared with the recently described Cosmarium regnesiforme (see that species for more details). The habitat is shallow moorland pools and bogs which are slightly acidic.

References: 
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium.
West, W. (1892). A contribution to the freshwater algae of west Ireland.