Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus triangularis var. indentatus P.F.M. Coesel & K.J. Meesters 2013

It differs from the nominate variety in having a slight indentation in the ventral sides. The isthmus is usually angular at its apex but can also be rounded (rounded and elongate isthmuses may suggest the start of division). The spines are usually parallel but occasionally slightly convergent.
Cell dimensions: L: 20-26 µm; B: 16-21.6 µm; Is: 4.8-8.4 µm; Th: 7.7 µm; Sp: 5-9 µm; Overall B: 30-47.
An acidophile found in moorland pools and bogs including with Sphagnum. Infrequently recorded.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.