Phylum: Charophyta   Family: Desmidiaceae

Xanthidium polymazum (C.F.O. Nordstedt) P.F.M. Coesel & K.J. Meesters 2023

Cells medium-sized with a closed sinus for about half its length, then opening widely. Semicells oblong-hexagonal. Paired spines occur at the lateral and apical angles, apex truncate. The cell-wall supports a subapical curved row of granules, which is diagnostic.
Cell dimensions (2): L: 56-59 µm; B: 53 µm; Is: 16.4-17.6 µm; Sp: 12-15 µm; Overall LxB: 80-82 x 78-84 µm.
A rare taxon with little information on habitat preference or status. New finds are awaited.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.