Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus validus  (West & G.S. West) Thomasson

It has a pantropical distribution. In Europe it is very rare and confined to Atlantic regions. Much larger than the variety with overall measurements in excess of 55-60 µm

References: 
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus
John, D.M., Whitton, B.A. & Brook, A.J. (2011 second edition) The Freshwater Algal Flora of the British Isles