Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus validus (W. & G.S. West) K. Thomasson 1960

This nominate variety is reportedly much larger than the var. subincus: West & West (1912: 96) give dimensions, minus spines, of 33-35 x 29-36 µm, but I haven’t found any cells approaching these sizes.
Cell dimensions: L: 27-29 µm; B: 22-24 µm.; Is: 8-9 µm; Sp: 24-31 µm; overall LxB: 56-67 x 66-70.
Infrequently found in slightly acidic waters such as in the plankton of lochs and moorland pools.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.