Phylum: Charophyta   Family: Desmidiaceae

Cosmarium pachydermum P.M. Lundell 1871

A large desmid with sub-semicircular semicells, thick-walled and densely punctate.
Single cell dimensions: L: 133 µm; B: 87 µm; Is: 31; L/B: 1.53.
This species would seem to be genuinely rare. The only Outer Hebridean find was from an upland, acidic, drainage bog with Sphagnum and other plants.

References: 
John, D.M. & Williamson, D.B. (2009). A Practical Guide to the Desmids of the West of Ireland.
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.