Phylum: Charophyta Family: Desmidiaceae
Cosmarium contractum var. minutum (Delponte) Coesel
It would seem to be genuinely rare and distribution little known. Found in similar habitats to the nominate
References:
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands