Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus validus var. subincus (W. & G.S. West) P.F.M. Coesel & K.J. Meesters 2013

The distinction between this variety and the nominate seems to be based almost exclusively on size. Coesel & Meesters (2013: 38) give for Std. validus anything greater than LxB: 60 x 55 µm. Anything less than these dimensions is var. subincus. There would seem to be considerable overlap between the two varieties, and many are found that are still developing their spines making a determination difficult.
Cell dimensions: L: 23-28 µm; B: 20-26 µm.; Is: 7-10 µm; Sp: 16-26 µm; LxB: 45-57 x 50-60.
Infrequently found in acidic waters such 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.