Phylum: Charophyta   Family: Desmidiaceae

Staurastrum longispinum (J.W. Bailey) W. Archer 1861

First described and illustrated by Bailey (1851: 36, pl. 1: 17) as Didymocladon longispinus. Shortly after Archer (1861: 743) moved it to its current genus. Typically found with two stout spines per lateral angle, but cells with three spines occur.
Cell dimensions: L: 80-130 µm; B: 70-135 µm; Is: 27-47 µm.
According to West, West & Carter (1923: 34) "It is very abundant in plankton." One hundred years on and it would seem to be rather scarce in the Outer Hebrides.

References: 
Archer, W. [in] Pritchard, A. (1871). A history of infusoria, including the Desmidiaceae and Diatomaceae, British and foreign.
Bailey, J.W. (1851). Microscopical observations made in South Carolina, Georgia and Florida.
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
West, W., West, G.S. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.