Phylum: Charophyta   Family: Desmidiaceae

Staurastrum monticulosum  Brébisson

An acidophile of mainly Arctic and boreal regions. Probably rare in the Outer Hebrides

References: 
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus
Carter, C.F. & Williamson, D.B. (2018) A contribution to the desmid (Charophyta, Conjugatophyta) flora of Newfoundland