Phylum: Charophyta   Family: Desmidiaceae

Cosmarium turpinii L.A. de Brébisson 1856

Semicells trapeziform, sinus partly closed at the apex to opening from the start. Lateral angles broadly rounded, upper part of side markedly divergent and retuse, narrow apex truncate. A distinct character is the adjacent twin protuberances which occasionally merge into one.
Cell dimensions: L: 72-76 µm; B: 63-70 µm; Is: 20-21.2 µm; Th: c.30-36 µm; L/B: 1.07-1.18.
Apparently quite common in circumneutral waters but more sampling is needed in the Outer Hebrides to establish its status.

References: 
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
Van Westen, M.C. (2024). Sieralgen in Drenthe.