Phylum: Charophyta   Family: Desmidiaceae

Cosmarium contractoideum  P.F.M. Coesel & K.J. Meesters 2023

Until recently, this was a variety of Cosmarium contractum, a much confused, complex. Because of the similarity with other taxa, identification can be difficult.
Cell dimensions: L: 32-38 µm, B: 26-34 µm, Is: 8-11.6 µm; Th: 14-18 µm, L/B: 1.0-1.2.
Because of identification difficulties, the status in the Outer Hebrides is uncertain.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Van Westen, M. (2024). Sieralgen in Drenthe.