Phylum: Charophyta   Family: Desmidiaceae

Cosmarium pseudannulatum F.A.C. Kouwets 2025

An unusual Cosmarium, oblong with a very small sinus. Semicells in face view are 5-6 undulate, based on Outer Hebridean finds. Each semicell has one pyrenoid.
Cell dimensions: L: 49-59 µm; B: 20-22 µm; Is: 19-20 µm; L/B: 2.3-2.9.
A rare desmid with an Arctic-alpine distribution, possibly with an Atlantic bias. Found predominantly on wet rocks but also reported from small pools. Even in ideal habitat, it's never abundant.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.