Phylum: Charophyta   Family: Desmidiaceae

Cosmarium subnudum (W. & G.S. West) P.F.M. Coesel & K.J. Meesters 2023

A large cell with a closed sinus for the most part with a slightly dilate apex. Semicells trapeziform, basal angle broadly rounded, sides slightly convex and convergent, apex truncate. About 8-10 granules are visible on either side of the semicell. It’s worth comparing it to the similarly shaped C. vexatiforme Kouwets 2025, but that species tends to be smaller.
Cell dimensions: L: 47-56 µm; B: 40-49 µm; Is: 11.6-17 µm; Th: 24-27 µm; L/B: 1.05-1.29.
West & West (1908: 186) have it as "… abundant among mosses on dripping rocks." This is the situation in the Outer Hebrides where, in such habitats, it can be remarkably abundant. It has also been found in slightly acidic, mesotrophic, loch-margins.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
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.