Phylum: Charophyta   Family: Desmidiaceae

Staurastrum anatinum var. denticulatum G.M. Smith 1924

First described and illustrated by Smith (1924: 95, pl. 75: 21-25). 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 three long spines.
Cell dimensions: L: 35-50 µm; B: 60-86 µm; Is: 10.5-12.5 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.