Phylum: Charophyta   Family: Desmidiaceae

Cosmarium ralfsii J. Ralfs 1848

A large cell with semicells sub-semicircular, the sinuses can be closed or partly open like a narrow edge. The cell-wall is covered with small pores. The chloroplast is axile with radiating ‘ledges’ and support a varying number of small pyrenoids.
Single cell dimensions: L. 114-128 µm; B. 99-106 µm; Is. 23-28.2 µm; Th. c.55 µm; L/B. 1.16-1.23.
Found in acidic moorland bogs, often with Sphagnum. West & West (1905: 142) note it “… is widely distributed in the upland districts of the British Islands, and often occurs in abundance among submerged Sphagnum.” Although there is ample habitat in the Outer Hebrides, it is proving to be rarely encountered.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
West, W. & West, G.S. (1905). A Monograph of the British Desmidiaceae, Volume 2