Phylum: Charophyta   Family: Desmidiacea

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

First described by Brébisson in Ralfs (1848: 126, pl. 21: 6 a-d) and illustrated by Jenner. This strange desmid has been hard to place taxonomically, and been shunted between at least three genera. Stastny, et al. (2013, p. 1) note: "Finally, we also found out that the morphologically peculiar Staurastrum tumidum Ralfs belongs to the genus Xanthidium …” However, Coesel & Meesters (2023: 251) still prefer to keep it as a Staurastrum, so I have followed their lead here.
Cell dimensions: L: 112-140 µm; B: 90-125 µm; Is: 44-71 µm.
A widespread but rare species found in acidic habitats.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Ralfs, J. (1848). The British Desmidieae.
Stastny, J. et al. (2013) Polyphasic evaluation of Xanthidium antilopaeum and Xanthidium cristatum species complex.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.