Phylum: Charophyta   Family: Desmidiaceae

Euastrum montanum W. & G.S. West 1905

West & West (1905: 17, pl. 1: 11-12) note: ‘This small Desmid is widely distributed in the upland districts of the British Islands. It has been known for the last ten years under the name of "Cosmarium Subreinschii, var. Boldtiana, Schmidle” but it is easily distinguished from Cosmarium Subreinschii by its larger and broader central protuberances, by its relatively wider and more angular apices, and by the apical notch … are features which at once place it in the genus Euastrum.'
Cell dimensions: L: 26.6 µm; B: 19.4 µm; Is: 6.7 µm; Th: 15 µm; B.apex: 13.1 µm; L/B: 1.38.
This is a very rare desmid, which has obviously declined since the Wests’ time. Found in acidic moorland pools and bogs.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
West, W. & West, G.S. (1905). Freshwater algae from the Orkneys and Shetlands.