Phylum: Charophyta   Family: Desmidiaceae

Euastrum pinnatum J. Ralfs 1848

There are two main characteristics that define this species and separate it from the similar Euastrum humerosum var. affine. A group of five protuberances: one basal with two others above forming a central triangle, and two others on the basal lobes. Also note the granulate cell-wall, most noticeable on the lobes. West & West (1905: 11), over 100 years ago, note: "in certain parts of west of Scotland and Ireland it occurs in abundance." More sampling is necessary to establish its current status in the Outer Hebrides but it would appear to be much scarcer than previously recorded.
Cell dimensions: L: 129-167 µm; B: 68-80 µm; Is: 19-26 µm; Th: 46-51 µm; L/B: 1.90-2.30.
Found in acidic moorland pools and bogs.

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.