Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus subhexagonus   (West & G.S. West) Coesel

A rare species found in acidic waters

References: 
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus