Phylum: Charophyta Family: Desmidiaceae
Cosmarium fastidiosum West & G.S. West
This species would seem to be genuinely rare. West & West (1908) only recorded it from a small pond in Oxfordshire. Its site preferences are not defined for Britain. For Europe, Tsarenko, et al. (2014) give "Planktonic, bethic in bogs, also in reservoirs; mesotrophic."
West, W. & West, G.S. (1908) A Monograph of the British Desmidiaceae, Volume 3
Tsarenko, P.M., Wasser, S.P. & Nevo, E. editors (2014) Algae of Ukraine, Volume 4: Charophyta
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands