Phylum: Charophyta   Family: Desmidiaceae

Staurastrum capitulum L.A. de Brébisson 1848

First described by Brébisson and illustrated by Jenner in Ralfs (1848: 214, pl. 35: 25). This species has an Arctic-alpine distribution and is not uncommon is these regions. It needs to be compared with S. pileolatum. It differs from that species in having its apical angles horizontally spread rather than upwardly directed.
Cell dimensions: L: 37-40 µm; B: 23-28 µm; Is: 13-14.6 µm.
Found with mosses on wet rocks and other wet substrates. Fairly common in these limited habitats.

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. (1912) A Monograph of the British Desmidiaceae, Volume 4.