Phylum: Charophyta   Family: Desmidiaceae

Micrasterias fimbriata J. Ralfs 1848

Similar to Micrasterias apicula but that species has a more ornamented polar lobe and more spines on the surface in face-view. West & West (1905: 99, as M. apicula var. fibriata) show a figure (l.c., pl. 46: 6) of an exaggerated curved polar spine very similar to the Outer Hebridean finds.
Cell dimensions (3): L: 225-245 µm; B: 210-220 µm; Is: 31-40 µm; L/B: 1.08-1.12.
This rare desmid has only been found on three occasions, all from two nearby lochs, which are mesotrophic with aquatics and slightly acidic.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
West, W. & West, G.S. (1905). A Monograph of the British Desmidiaceae, Volume 2.