Phylum: Charophyta Family: Desmidiaceae
Staurastrum sexangulare (Bulnheim) Rabenhorst 1863
The West, West & Carter (1923, p. 194, pl. 157: 2-3) describe its distributions as 'not an uncommon species in the plankton of somes lakes in the western part of the British Isles, but otherwise very rare.' One hundred and twenty years later, it's proving scarce even in the west.
References:
West, W., West, G.S. & Carter, N. (1923) A Monograph of the British Desmidiaceae, Volume 5
John, D.M. & Williamson, D.B. (2009) A Practical Guide to the Desmids of the West of Ireland
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus