Phylum: Charophyta   Family: Desmidiaceae

Cosmarium decedens  (Reinsch) Raciborski

An arctic/alpine species which is rare in Britain, confined to acidic boggy ground in the north and west. The cell-wall is variously described as "smooth" - Coelsel & Meesters (2007); "delicately punctate" - West & West (1908) and "with coarse pores" - John & Williamson (2009), all noted authors suggesting a variable character

References: 
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands
West, W. & West, G.S. (1908) A Monograph of the British Desmidiaceae, Volume 3
John, D.M. & Williamson, D.B. (2009) A Practical Guide to the Desmids of the West of Ireland