Phylum: Charophyta   Family: Desmidiaceae

Cosmarium ridibundum (W.R. Taylor) F.A.C. Kouwets 2025

This large cell has convex sides rather that truncate like C. margaritatum 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: 80-87 µm; B: 62-67 µm; Is: 23- 26.3 µm; Th: c.41 µm; L/B: 1.23-1.38.
This would seem to be a rare Arctic-Atlantic taxon. The early Outer Hebridean finds have come from slightly alkaline habitats (pH 7.2-7.4) with minimal vegetation. Recently (2025), a healthy population was found in a mesotrophic, slightly acidic, moorland lochan with aquatics.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.