Phylum: Charophyta   Family: Closteriaceae

Closterium incurvum  L.A. de Brébisson 1856

First described and illustrated by Brébisson (1856: 150, pl. 2: 47) who recorded it as rare. West & West (1904: 136, pl. 15: 28-30) describe it as a rare desmid in Britain with no mention of the Outer Hebrides.
Cell dimensions: L: 40-80 µm; B: 6-12 µm; Ap: 1.0-2.0 µm; Arc: 180; L/B: 5-7.
It apparently favours a range of habitats with varying degrees of acidity but infrequently recorded.

References: 
Brébisson, L. A. de (1856) Liste des Desmidiées, observées en Basse-Normandie.
Brook, A.J. & Williamson, D.B. (2010) A Monograph on some British Desmids.
Coesel, P.F.M. & Meesters, K.J. (2023 second edition) Desmids of the Lowlands.
West, W. & West, G.S. (1904) A Monograph of the British Desmidiaceae, Volume 1.