Phylum: Charophyta Family: Closteriaceae
Closterium cornu Ehrenberg ex Ralfs 1848
An acidophile found in peat bogs, disused peat-cuttings and especially with Sphagnum. It's proving to be very scarce, if not rare, in the Outer Hebrides.
References:
Brook, A.J. & Williamson, D.B. (2010) A Monograph on some British Desmids.
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands.
West, W. & West, G.S. (1904) A Monograph of the British Desmidiaceae, Volume 1.