Phylum: Charophyta   Family: Desmidiaceae

Staurastrum anatinum var. denticulatum G.M. Smith 1924

It differs from the nominate in having more pronounced ornamentation. Seen best in face view where the apex has a series of verrucae. The processes have long spines especially near the cell-body and tipped with four stout spines.
Cell dimensions: L: 35-37 µm; B: 31-33 µm; Is: 13-14 µm; Overall LxB: 46-61 x 66-83 µm.
Apparently, this variety is more often found in circumneutral waters. The status is unclear for the Outer Hebrides as it’s infrequently recorded.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Smith, G.M. (1924). Phytoplankton of the Inland Lakes of Wisconsin. Part II Desmidiaceae.