Phylum: Charophyta   Family: Desmidiaceae

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

First described and illustrated by W. & G.S. West (1894: 11, pl. 1: 22). 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.”
Cell dimensions: L: 15-30 µm; B: 15-30 µm; Is: c.6 µm.
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: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
West, W. & West, G.S. (1894). New British freshwater algae.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.