Phylum: Charophyta Family: Desmidiaceae
Cosmarium vexatum West 1892
This fits best with Cosmarium vexatum var. concavum Schmidle but that variety has been subsumed into the problematic C. vexatum. Lenzenweger (1999, p. 124, pl. 55: 10) describes and illustrates the variety well. West & West (1908, p. 187, pl. 92: 4) describe it as locally abundant from a site in Westmorland (now Cumbria), otherwise presumably rare. Some of the images pictured here are a little on the small size according to most authors.
References:
Lenzenweger, R. (1999) Desmidiaceenflora von Österreich, Teil 3
West, W. & West, G.S. (1908) A Monograph of the British Desmidiaceae, Volume 3
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands