Phylum: Charophyta   Family: Desmidiaceae

Actinotaenium obcuneatum  (W. West) E. Teiling 1954

Originally described as Cosmarium obcuneatum West 1892 from the west of Ireland. By 1908, West & West note: “There is no doubt that C. obcuneatum is identical with C. parvulum.” C. parvulum Brébisson 1856 is poorly described, which has led Kouwets (2025) to describe better known material as C. parvuloides. Because of the confusion with the genus Cosmarium the wide distribution in Britain given by the Wests may not relate to the taxon under discussion. The basal angles are slightly produced with more or less straight lateral sides; circular in apical view.
Cell dimensions: L: 32-36 µm; B: 15.9-19 µm; Is: 15-17.2 µm; L/B: 1.82-2.12.
A rare desmid but possibly confused with Cosmarium (see above). The only sample from the Outer Hebrides came from squeezed bryophytes.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.
West, W. & West, G.S. (1908). A Monograph of the British Desmidiaceae, Volume 3.