Phylum: Charophyta   Family: Closteriaceae

Closterium dianae var. arcuatum  (Brebisson ex Ralfs) Rabenhorst 1868

Differs from the nominate variety by have a stronger curvature but the margins can be small making separation difficult. Found in peaty pools, including with Sphagnum, where it is not uncommon.

References: 
Brook, A.J. & Williamson, D.B. (2010) A Monograph on some British Desmids.
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.