Phylum: Charophyta   Family: Desmidiacea

Xanthidium planum (Y.V. Roll) P.F.M. Coesel & K.J. Meesters 2023

First described and illustrated by Roll (1936: 9, pl. 1: 10–12, pl. 2: 17) as a variety of Xanthidium antilopaeum. Recently, Coesel & Meesters (2023: 195) raised it to species level. Its main feature is having the spines curved downwards. It has a central caldera with 8-10 smaller ones surrounding it. This is so similar to Xanthidium antilopaeum var. ornatum Borge, that it may well be synonymous.
Cell dimensions: L: 47-66 µm; B: 45-61 µm; Is: 16-26 µm; Sp: 10-17 µm; Overall LxB: 74-89 x 77-88 µm.
Found in acidic mesotrophic waters such as moorland pools and shallow lochans, but infrequently recorded.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Roll, Y.V. (1936). Materialy dlia flory vodorostey SRSR [Materials for the algological flora of the USSR].
Van Westen, M. (2024). Sieralgen in Drenthe.