Phylum: Charophyta   Family: Desmidiaceae

Cosmarium speciosum P.M. Lundell 1871

This quite large desmid has semicells best described as depressed pyramidate. Sinus closed for the most part, sides granular-undulate with 16-20 undulations visible in face-view per semicell, apex truncate. This is probably a species complex with much taxonomic drift from earlier authors. Compare with the slightly smaller C. speciosissimum Schmidle 1895.
Cell dimensions: L: 53-62 µm; B: 39-44 µm; Is: 20-22.5 µm; Th: 26-27.1 µm; L/B: 1.38-1.42.
It seems to prefer sub-atmophytic, slightly acidic, conditions, also ephemeral water. It's infrequently recorded in the Outer Hebrides.

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