Phylum: Charophyta   Family: Desmidiaceae

Cosmarium contractum  O. Kirchner

Found in acidic moorland pools and bogs. The current status for the Outer Hebrides is that it's scarcer than some of the varieties

References: 
John, D.M. & Williamson, D.B. (2009) A Practical Guide to the Desmids of the West of Ireland
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands
West, W. & West, G.S. (1905) A Monograph of the British Desmidiaceae, Volume 2