Phylum: Charophyta   Family: Desmidiaceae

Cosmarium davidsonii J. Roy & J.P. Bisset 1894

Semicells trapeziform with about parallel sides. The sides are 3-4 undulate with the apex 4 undulate including the apical angles, which are slightly produced. In apical view there is a noticeable swelling.
Single cell dimensions: L: 31.8 µm; B: 22 µm; Is: 13.1 µm; Th: 16.4 µm; L/B: 1.45.
In Europe it is widespread if not particularly common. In the Outer Hebrides its status is unknown due to possible taxonomic confusion. The preferred habitat is ephemeral pools and mosses on wet rocks.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
Van Westen, M.C. (2024). Sieralgen in Drenthe.