Phylum: Charophyta   Family: Desmidiaceae

Staurastrum sexcostatum L.A. de Brébisson ex J. Ralfs 1848

First described by Brébisson in Ralfs (1848: 129, pl. 23: 5) and illustrated by Jenner. Ralfs (l.c.) describes it as “very rare” for Britain, but it’s more frequent in the Outer Hebrides. As the name suggests, cells are predominantly 6-radiate.
Overall cell dimensions: L: 38-50 µm; B: 35-45 µm; Is: 12-20 µm.
The main habitat is acidic shallow loch-margins, pools and ditches, including with Sphagnum.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Ralfs, J. (1848). The British Desmidieae.
West, W., West, G.S. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.