Phylum: Charophyta   Family: Desmidiaceae

Cosmarium arcuatiporosum M.C. van Westen & P.F.M. Coesel 2020

This recently described species resembles C. subtumidum in outline shape but C. arcuatiporosum has a seemingly unique pore-pattern. There is a curved row of pores above the isthmus and a pore-field at the apices. Between these pores is an unsculptured zone. This shows well in SEM photographs: see Van Westen & Coesel (2020: 111, fig. 27).
Cell dimensions: L: 24-25.7 µm; B: 19-21 µm; Is: 8-9.6; Th: c.12-13.4; L/B: 1.2-1.25.
This is of rare occurrence in the Outer Hebrides but may have been confused with similar species in the past. This recent find was in mesotrophic, circumneutral waters.

References: 
Van Westen, M.C. & Coesel, P.F.M. (2020). Taxonomic notes on desmids from the Netherlands III, with a description of five new species.