Phylum: Charophyta   Family: Desmidiacea

Xanthidium antilopaeum var. leave   Schmidle

Differs from the nominate by the wide open sinus and rounded basal angles. It's a large cell and the one depicted is at the lower end of the range. It's proving to be quite scarce

References: 
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands West, W. & West, G.S. (1912) A Monograph of the British Desmidiaceae, Volume 4