Phylum: Charophyta   Family: Desmidiaceae

Euastrum crassum J. Ralfs 1848

Semicells broadly pyramidal with straight to slightly incurved sides. Apical and lateral lobes separated by deep incisions. The lateral lobe is occasionally separated from the apical lobe and can have a rounded or broadened apex. Basal angles and apical lobes thickened. Sinus deep and closed. Either side of the isthmus are three pronounced protuberances. Cell wall marked with medium pores and at least one deep scrobiculum.
Cells are frequently found with the laterals supporting additional median rounded protuberances. These protuberances are erratic: occasionally showing on one side of a cell or just on one semicell. This feature has long been observed: Harris (1928, No.14: 8, pl. 5: 2) suggested it should be made a variety but it was never validly published. This was at a time when any slight variation was deemed suitable for varietal of forma status. The feature described here is found in mixed populations with cells not displaying the feature.
Cell dimensions: L: 140-210 µm; B: 70-110 µm; Is: 17-40 µm; Th: 40-75 µm; L/B: 1.75-2.10.
Found in acidic conditions such as Sphagnum bogs and moorland pools. Infrequently found in the Outer Hebrides, which is surprising as there is ample habitat.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition) Desmids of the Lowlands.
Harris, G.T. (1928) Watson’s Microscope Record.
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.