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: 26-28 µm; B: 22-24 µm.; Is: 7-9 µm; Sp: 19-32 µm; overall LxB: 56-68 x 47 -71.
Infrequently found in slightly acidic waters such as in the plankton of lochs and moorland pools.

References: 
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.