Phylum: Charophyta   Family: Desmidiaceae

Staurastrum ralfsii var. depressum (J. Roy & Bisset) Coesel & Meesters 2013

Differs from the nominate in having the length and breadth about equal, and smaller cells. Found in oligo-mesotrophic waters with a wide range of pH values. Roy & Bisset (1886) first described this as a variety of Staurastrum orbiculare.

References: 
Roy, J. & Bisset, J.P. (1886) Notes on Japanese desmids. - No. I
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus