Phylum: Charophyta   Family: Desmidiaceae

Cosmarium subcostatum C.F.O. Nordstedt 1876

This medium-sized desmid has semicells best described as depressed trapeziform. Sinus closed for the most part with a dilated apex, basal angles broadly rounded with the sides undulate and converging, apex slightly produced and slightly undulate. The undulations caused by granules, those nearest the apex are emarginate.
Cell dimensions (2): L: 35.1-40 µm; B: 32.9-33.9 µm; Is: 10.3-11.7 µm; Th: 6c.20 µm; L/B: 1.07-1.18.
A rare desmid with its status in the Outer Hebrides unknown. The only find to date (2025) was from a squeeze of Myriophyllum alterniflorum from an alkaline loch-margin.

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