Phylum: Charophyta   Family: Desmidiaceae

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

First described by Brébisson in Ralfs (1848: 215, pl. 35: 24) and illustrated by Jenner. Found in triradiate and quadriradiate form.
Cell dimensions: L. 30-42 µm, B. 27-45 µm, Is. 10-12 µm.
An acidophile that is widespread in Europe but rare. In the Outer Hebrides, only one collection has been made, where it was abundant. The population from Benbecula (NF8396 4791) was predominantly quadriradiate. I'm grateful to Marien van Westen for the identification from this sample.

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.