Phylum: Charophyta   Family: Desmidiaceae

Staurastrum pelagicum  West & G.S. West

A very rare species in Britain with records from Orkney, Shetland and Ireland. This current population is from a small loch just on the acidic side of neutral. Some specimens have an extra spine which I haven't seen described anywhere

References: 
West, W., West, G.S. & Carter, N. (1923) A Monograph of the British Desmidiaceae, Volume 5
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus