Phylum: Charophyta   Family: Desmidiacea

Cosmarium margaritatum var. ridibundum W.R. Taylor 1934

This large cell differs from the nominate variety in having convex sides rather that truncate and supports large granules with 24 visible around the periphery of the semicell. A remarkable feature is the row of granules immediately above the isthmus, with many truncate and emarginate, which form a shallow arc.
Cell dimensions: L: 84-85 µm; B: 66-67 µm; Is: 23- 25.3 µm; L/B: 1.27-1.28.
This would seem to be a rare Arctic-Atlantic taxon. The Outer Hebridean finds have come from slightly alkaline habitats (pH 7.2-7.4) with minimal vegetation.

References: 
Kouwets, F.A.C. (in manuscript). European Flora of the Desmid Genus Cosmarium.
Taylor, W. R. (1934). The fresh-water algae of Newfoundland, Part I.