Phylum: Charophyta   Family: Desmidiaceae

Staurastrum lanceolatum var. compressum West & G.S. West 1894

In their later flora, West & West (1912, p. 150, pl. 121: 7) distinguish this variety from the nominate by: 'Cells somewhat compressed, sinus narrower especially at the extremity; apex of semicells flattened; sides of vertical view more distinctly concave.' This taxon is particularly rare but, interestingly, the Wests give Lewis, Outer Hebrides as one of two sites. Most probably an Atlantic-arctic distribution.

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.