Phylum: Charophyta   Family: Desmidiaceae

Staurastrum polymorphum L.A. de Brébisson ex J. Ralfs 1848

First described by Brébisson in Ralfs (1848: 135, pl. 22: 9) and illustrated by Jenner. The epithet has become an invitation to broaded its parameters and become a catch-all species. However, the original diagnosis is sound. The spiny tips to the processes and the intramarginal bifurcate granules in apical view are good identification features.
Cell dimensions: L: 20-30 µm; B: 31-40 µm; Is: 8-10 µm.
Frequently found in peaty pools and bogs but less often in the plankton.

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. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.