Phylum: Charophyta   Family: Desmidiaceae

Cosmarium pseudotumidum P.F.M. Coesel & K.J. Meesters 2023

Slightly larger than C. tumidum P.M. Lundell 1871, especially regarding the isthmus. It has wall-thickening at the lateral angles and a closed sinus for half its length with a slightly dilate apex.
Cell dimensions (2): L: 37.9-38.3 µm; B: 30.9-31.5 µm; Is: 11.8-12.6 µm; Th: c.20; L/B: 1.22-1.23.
Because of frequent misidentifications with similar taxa, the status in the Outer Hebrides is unknown. Current finds are from a mesotrophic loch with little vegetation.

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.