Phylum: Charophyta Family: Desmidiaceae
Staurastrum longispinum (Bailey) W. Archer 1861
According to the Wests & Carter (1923) "It is very abundant in plankton." One hundred years on and it would seem to be rather scarce in the Outer Hebrides. Typically found with two stout spine per lateral angle, but cells with three spines occur.
References:
John, D.M. & Williamson, D.B. (2009) A Practical Guide to the Desmids of the West of Ireland
West, W., West, G.S. & Carter, N. (1923) A Monograph of the British Desmidiaceae, Volume 5
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus