Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus incus  (Hassal ex Ralfs) Teiling 1967

Frequent in the margins of nutient-poor lochs, and pool. Variable in size: those cells with the shortest spines seem to have the broadest isthmus.

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
Tsarenko, P.M., Wasser, S.P. & Nevo, E. editors (2014) Algae of Ukraine, Volume 4: Charophyta