Phylum: Charophyta   Family: Closteriaceae

Closterium lunula J. Ralfs 1848

One of the largest cells in the genus and can often be found in abundance in suitable habitats. Apices are rounded and often reflexed. There is considerable morphological variation between and within populations. Cells can also display variation as they rotate under coverslips. Var. biconvexum is noted for its equally convex side but extreme caution is required as cells can portray such images as they rotate under the coverslips, such as my top image in the final micrograph. There must be considerable doubt as to the validity of that latter variety.
Cell dimensions: L: 603-711 µm; B: 96-110 µm; Ap: 20-22 µm; L/B: 6-7.
Widespread and can be locally common in acidic peaty pools and bogs, especially with Sphagnum. West & West (1904: 150) specifically mention it being 'general' in the Outer Hebrides.

References: 
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.