Phylum: Charophyta   Family: Demidiacea

Cosmarium polymazum (C.F.O. Nordstedt) F. Wolle 1884

This species is characterised by a row of three large protuberances and a supra-isthmial tubercle. It needs to be compared with the morphologically similar-looking C. monomazum Lundell 1871 but can easily be separated on different central ornamentation.
Cell dimensions: L: 35.4-36.2 µm; B: 34.6-37.6 µm; Is: 11.4-14; Th: 19.6-22.4 µm; L/B: 0.96-1.03.
It would seem to be widespread but infrequently encountered in the Outer Hebrides. However, when found it can be locally abundant. The ecology is silty loch-margins in circumneutral waters.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.