Phylum: Charophyta   Family: Desmidiaceae

Staurastrum orbiculare J. Ralfs 1848

The semicell is about semicircular whereas the similar S. ralfsii is more pyramidate, it is also smaller, but there is overlap.
Cell dimensions: L: 40-62 µm; B: 34-52 µm; Is: 10-13 µm.
Widespread throughout Europe but more common in the Atlantic north-west. An acidophile but tolerant of circumneutral waters.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Ralfs, J. (1848). The British Desmidieae.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.