Phylum: Charophyta   Family: Desmidiaceae

Cosmarium tatricum M. Raciborski 1885

This has a very similar shape and size to Cosmarium decedens (Reinsch) Raciborski 1889. Cosmarium tatricum has a narrower apex in relation to the breadth of the semicells’ base. It also has a lower L/B ratio, but there is overlap. Intermediates exist which are difficult to determine.
Cell dimensions: L: 46-54 µm; B: 23-30 µm; Is: 15.8-20.8 µm; Th: 19.3-21.7 µm; L/B. 1.62-2.00.
An Arctic-alpine species that prefers sub-atmophytic habitats such as wet rocks. Not particularly common in the Outer Hebrides.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.
Van Westen, M.C. (2024). Sieralgen in Drenthe.