Phylum: Charophyta   Family: Desmidiaceae

Euastrum humerosum J. Ralfs 1848

Semicells pyramidal with double basal lobes and a lateral lobe above the median line. Both basal and lateral lobe can vary between specimens by how pronounced they are. Apices truncate with a deep, closed, median incision. In face view there are 5 protuberances, three across the base of the semicell, and two across the centre. At the base of these two is a noticeable scrobiculum.
Cell dimensions: L: 120-150 µm; B: 65-85 µm; Is: 13-30 µm; Th: 35-58; L/B: 1.70-1.90.
This is very similar to E. didelta (see that species for more information), intermediates exist that are difficult to identify. An acidophile of moorland pools and bogs. Infrequently recorded in the Outer Hebrides, which has ample suitable habitat.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition) Desmids of the Lowlands.
Ralfs, J. (1848) The British Desmidieae.
Ruzicka, J. (1981) Die Desmidiaceen Mitteleuropas, Volume 1, Part 2.
West, W. & West, G.S. (1905) A Monograph of the British Desmidiaceae, Volume 2.