Phylum: Charophyta   Family: Desmidiaceae

Cosmarium vexatum West

This fits best with Cosmarium vexatum var. concavum Schmidle but that variety has been subsumed into the problematic C. vexatum. Lenzenweger (1999) describes and illustrates the variety well. The Wests (1908) describe it locally abundant from a site in Westmoreland, otherwise presumably rare

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