Phylum: Charophyta   Family: Desmidiaceae

Staurastrum anatinum var. denticulatum G.M. Smith 1924

A large cell with an open, acute-angled sinus, semicells subfusiform, occasionally more bowl-shaped. Processes are divergent, sometimes just marginally so, tipped with usually 3 stout spines. Apices are truncate to slightly convex and support simple and emarginate teeth. In apical view predominantly 3-radiate with about straight sides. It differs from the nominate in having more pronounced ornamentation, especially on the processes.
Overall cell dimensions: L: 49-57 µm; B: 77-83 µm; Is: 13.2-14.2 µm; L/B: 0.64-0.69.
Apparently, this variety is more often found in circumneutral waters. The status is unclear for the Outer Hebrides as it is 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.