Phylum: Charophyta   Family: Desmidiaceae

Cosmarium subcrenatum C.A. Hantzsch 1868

Cosmarium crenatum and C. subcrenatum are problematic species, which were poorly described. They probably represent a species complex. Frans Kouwets (in manuscript) is attempting to sort out the confusion. I have shown Frans these images, which I think belong to his unpublished Cosmarium troumousense, which he thinks most likely. They will have to remain here until his work is published. "C. troumousense is only known from a few localities in the French Pyrenees. It apparently prefers sub-atmophytic habitats such as moss cushions in (spring) bogs and dripping rocks. It probably is an arctic-alpine form with a much wider distribution." - Kouwets (in manuscript).
Cell dimensions from West, W. & West, G.S. (1908: 229): L: 23-37 µm; B: 18-30 µm; Is: 8-14 µm; Th: 12-19 µm.
The images from a granite depression at Ardivachar differ slightly from the Isle of Lewis population but fit the Wests’ broader concept. More finds are needed to better evaluate this complex.

References: 
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium.
West, W. & West, G.S. (1908) A Monograph of the British Desmidiaceae, Volume 3.