Phylum: Charophyta   Family: Desmidiaceae

Cosmarium brebissonii  J. Ralfs 1848

A large taxon with stout, pointed, granules (almost teeth-like). It needs to be carefully compared with similar large, granular, taxa; see Kouwets (2025: 112) for more details.
Cell dimensions: L. 95-108 µm; B. 70-80 µm; Is. 20-34 µm; Th. 58-60 µm; L/B. 1.25-1.44.
Found in acidic habitats, often in association with Sphagnum. West & West (1908: 161-162) specifically note “Common in the Outer Hebrides”. It remains quite common here.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
West, W. & West, G.S. (1908). A Monograph of the British Desmidiaceae, Volume 3.