Phylum: Charophyta Family: Desmidiacea
Xanthidium antilopaeum var. laeve Schmidle 1893
Differs from the nominate by the wide open sinus and rounded basal angles. The spines are relatively short and stout. It's proving to be quite scarce in the Outer Hebrides.
References:
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands.
West, W. & West, G.S. (1912) A Monograph of the British Desmidiaceae, Volume 4.
Prescott, W.G. et al. (2001 reprint) A Synopsis of North American Desmids. Part II, Desmidiaceae: Placodermae, Section 4.