Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus convergens  (Ehrenberg ex Ralfs) S. Lillieroth

Common in shallow, slightly acid, mesotrophic water. The length of the spines varies considerably and the cell-shape alters accordingly

References: 
West, W. & West, G.S. (1912) A Monograph of the British Desmidiaceae, Volume 4
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus
John, D.M. & Williamson, D.B. (2009) A Practical Guide to the Desmids of the West of Ireland