Phylum: Charophyta   Family: Desmidiaceae

Xanthidium tumidum (Ralfs) Stastny, Skaloud & Neustupa 2013

This strange desmid has been hard to place taxonomically, and been shunted between at least three genera. Stastny, et al. (2013, p. 1) have probably found its final resting place with Xanthidium. They note: "Finally, we also found out that the morphologically peculiar Staurastrum tumidum Ralfs belongs to the genus Xanthidium based on the combined rbcL + cox III data set. Consequently, this species was formally transferred to this genus." A widespread but rare species found in acidic habitats.

References: 
Ralfs, J. (1848) The British Desmidieae.
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Stastny, J. et al. (2013) Polyphasic evaluation of Xanthidium antilopaeum and Xanthidium cristatum species complex.