Phylum: Charophyta Family: Desmidiaceae
Cosmarium calculus Coesel
Recently described but may prove to be not too uncommon in acidic habitats with vegetation. There are a number of small species with smooth walls to cause confusion
References:
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium