Phylum: Charophyta   Family: Desmidiaceae

Cosmarium calculus P.F.M. Coesel 2007

A recently described species similar to several other small, smooth-walled species. Much confusion exists between them and more finds with zygospores are required.
Cell dimensions: L: 10.4-13.7 µm, B: 10.9-12.9 µm, Is: 3.6-4.5 µm; Th: 5-7 µm, L/B: 0.96-1.07.
Found in slightly acidic, mesotrophic abandoned peat-cuttings. Distribution in the Outer Hebrides is unclear as more sampling is required.

References: 
Coesel, P.F.M. (2007). Taxonomic notes on Dutch desmids IV: new species, new names, new combinations.
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.
Van Westen, M. (2024). Sieralgen in Drenthe.