Phylum: Charophyta   Family: Desmidiaceae

Cosmarium heimerlii W. & G.S. West 1895

A very small cell with the sinuses closed for the most part. Semicells are depressed oblong-rectangular with the basal angles broadly rounded, upper lateral angles usually with a small granule, but this can be rudimentary or even missing. In the centre of the semicell is a small papilla but this too can be rudimentary. Apices truncate to slightly depressed. The chloroplast is axile with a single pyrenoid.
Cell dimensions (3): L: 8.6-9.6 µm, B: 9-10.8 µm, Is: 2.8-3 µm; Th: c.5-6 µm; L/B: 0.89-0.96.
Found in acidic, boggy habitats, often with Sphagnum. Only one site has been identified so far (2025) in the Outer Hebrides.

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