Phylum: Charophyta   Family: Desmidiaceae

Xanthidium variabile (C.F.O. Nordstedt) W. & G.S. West 1990

A small cell with rectangular to trapezoid semicells. Sinuses wide-open and wedge-shaped, lateral and apical angles support short spines. The central papillate protuberance often supports a tooth-like granule, occasionally emarginate.
Cell dimensions: L: 21-23 µm; B: 18-23 µm; Is: 8.5-9.2 µm; Sp: 1.2-3.4 µm; Th: 14.1 µm; overall LxB: 22-26 x 24-28.
An acidophile mainly from moorland pools and bogs, especially with Sphagnum. It's proving to be rather scarce in the Outer Hebrides even though we have ideal habitat.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
John, D.M. & Williamson, D.B. (2009). A Practical Guide to the Desmids of the West of Ireland.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.