Phylum: Charophyta   Family: Desmidiaceae

Cosmarium ochthodes Nordstedt 1875

Its status in the Outer Hebrides is uncertain as it's easily confused with C. botrytis and its varieties. It has a larger isthmus breadth and a larger L/B ratio, but these measurements overlap. Possibly inhabiting slightly more acidic waters than C. botrytis but they have been found together in the same sample.

References: 
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands.
John, D.M. & Williamson, D.B. (2009) A Practical Guide to the Desmids of the West of Ireland.
West, W. & West, G.S. (1912) A Monograph of the British Desmidiaceae, Volume 4.