Phylum: Chlorophyta   Family: Selenestraceae

Ankistrodesmus falcatus  (A.J.C. Corda) J. Ralfs 1848

First described and illustrated by Corda (1835: 206, pl. II: 29), rather surprisingly, as Micrasterias falcatus. It was later moved to its current taxonomic position by Ralfs (1848: 180, pl. 34: figs a-c).
Cell dimensions: L: 20-165 µm; B: 1.5-7 µm.
A fairly common planktonic species, found in loch-margins, moorland pools and also in running water.

References: 
Corda, A.J.C. (1835). Observations sur les animalcules microscopiques, qu'on trouve auprès des eaux thermales de Carlsbad.
John, D.M., Whitton, B.A. & Brook, A.J. (2011 second edition). The Freshwater Algal Flora of the British Isles.
Ralfs, J. (1848). The British Desmidieae.