Phylum: Charophyta   Family: Desmidiaceae

Staurastrum muticum J. Ralfs 1848

For a relatively small desmid, it experiences a considerable size-range, especially now forma minus Rabenhorst 1868 has been subsumed into the nominate variety. It is likely that more than one taxon is involved.
Coesel & Meesters’ (2013) cell dimensions: L: 15-45 µm; B: 15-40 µm.
Infrequently recorded in the Outer Hebrides. Found in the benthos of acidic habitats such as moorland pools and bogs.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.