Phylum: Charophyta   Family: Desmidiaceae

Euastrum insulare  (B. Wittrock) J. Roy 1877

First described by Wittrock (1872: 49, pl. 4: 7) as a variety of E. insulare. Roy (1877: 70) raised it to species level. Lower half of semicells rectangular with two small lobes, upper half also rectangular but narrower. Apex with a shallow dent. Sinus with a dilate apex else closed for the most part. This species has a narrower isthmus compared to similar taxa.
Cell dimensions: L: 20-27 µm; B: 14-18 µm; Is: 3.5-5.5 µm; Th: 9.5–11.5 µm; L/B: 1.20-1.50.
West & West (1905: 68, pl. 40: 11-13) describe it as widespread and include the Outer Hebrides in their list. Today, it doesn't occur so frequently. Found with Sphagnum and in less acidic habitats.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition) Desmids of the Lowlands.
Roy, J. (1877) Contributions to the demid flora of Perthshire. No. 1.
West, W. & West, G.S. (1905) A Monograph of the British Desmidiaceae, Volume 2.
Wittrock, B. (1872) Om Gotlands och Ölands sötvattens-alger.