Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus convergens (J. Ralfs) S. Lillieroth 1950

A medium-sized cell, which is sometimes referred to as polymorphic. However, I find it only changes shape with age. Young semicells are subrectangular with the sinuses closed for half their length and with a burgeoning spine. Mature cells are elliptical with the sinuses open from the start and with long, decumbent, spines. A biradiated species.
Cell dimensions: L: 39-51 µm; B: 43-57 µm; Is: 10.6-15.8 µm; Sp: 3-15 µm; overall B: 43-85.
Common in shallow, slightly acid, mesotrophic water, also in the tychoplankton.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Teiling, E. (1967). The Desmid Genus Staurodesmus. A taxonomic study.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.