Phylum: Charophyta   Family: Closteriaceae

Closterium angustatum var. sculptum (M. Raciborski) J. Ruzicka 1977

It differs from the nominate in that the striae (costae) have the crests interrupted with thickenings giving a corrugated or knotted appearance. The apices are swollen to varying degrees giving a capitate appearance.
Cell dimensions: L: 230-416 µm; B: 10-22 µm; Ap: 6.5-13.5 µm; L/B: 16-23.
Current finds from the Outer Hebrides were in slightly acidic, mesptrophic, waters. It would appear to be geographically scarce here but can be locally abundant.

References: 
Brook, A.J. & Williamson, D.B. (2010). A Monograph on some British Desmids.
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.