Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus extensus var. isthmosus   (Heimerl) Coesel

Common in similar habitats to the type species

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) The Freshwater Algal Flora of the British Isles