Phylum: Charophyta   Family: Desmidiaceae

Cosmarium sphaeroideum  W. West 1892

Similar to C. amoenum Ralfs 1848 but that species is a little smaller, especially in breadth with a higher L/B: ratio.
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.
An infrequently found species with the British population seemingly confined to the granitic regions of Scotland and Ireland. An acidophile largely confined to moorland bogs usually with Sphagnum.

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