Phylum: Charophyta   Family: Demidiacea

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

First described and illustrated by Nordstedt (1873: 14, pl. 1: 3) as a variety of C. monomazum. C. polymazum has a more elaborate ornamentation and there are no known intermediates persuading Wolle (1884: 70, pl. 16: 38-40) to raised it to species rank. This species is characterised by a row of three large protuberances and a supra-isthmial tubercle.
Cell dimensions: L: 33-39 µm; B: 33-39 µm; Is: 10-15; Th: 19-23 µm; L/B: 0.93-1.04.
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. (in manuscript) European Flora of the Desmid Genus Cosmarium.
Nordstedt, C.F.O. (1873) Bidrag till kännedomen om sydligare Norges Desmidiéer.
Wolle, R. (1884) Desmids of the United States.