Phylum: Charophyta   Family: Desmidiaceae

Cosmarium regnesiforme Van Westen & Coesel 2021

This recently described desmid is very similar to Cosmarium regnesi var. tritum West 1892. Both are triangular at the base of the semicells but differs in having noticeably large granules on the lateral margins and also on the apex. It is also smaller overall, with some overlap with the length and breadth but none with the thickness and breadth of isthmus. The L/B ratio is also smaller. This will have been confused with similar species in the past so distribution in the Outer Hebrides is unknown. It has been found in slightly acidic, oligo-mesotrophic waters.

References: 
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium.
Van Westen, M.C. & Coesel, P.F.M. (2021) Taxonomic notes on desmids from the Netherlands IV, with a description of another five new species.
West, W. (1892) A contribution to the freshwater algae of west Ireland.