Phylum: Charophyta   Family: Desmidiaceae

Cosmarium homalodermum C.F.O. Nordstedt 1875

Semicells subtrapeziform, sinus slightly dilate at the apex, closed for the most part then opening widely. Basal angles broadly rounded, lateral sides straight or slightly convex, apex more or less truncate. All angles can show wall-thickening: when the apical angles are thickened it gives the impression that the apex is retuse. Chloroplast axile with one large pyrenoid.
Cell dimensions (3): L: 63-68.5 µm, B: 50-51.7 µm, Is: 17.8-20.9 µm; Th: 32.3-34.5 µm, L/B: 1.27-1.34.
A widespread but rare desmid found in spring moorland pools and with moss on wet rocks. The Outer Hebridean finds have come from squeezed moss on wet rocks.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.