Phylum: Charophyta   Family: Desmidiaceae

Xanthidium controversum var. planctonicum W. & G.S. West 1903

This rare taxon differs from the nominate variety in being larger with a dilated extremity to the sinuses. The sinuses can be partly closed to open from the start depending on the shape of the semicell-bases. Semicells are elliptic-hexagonal with the centres scrobiculate with a shallow central protuberance. The four single spines (occasionally paired) occur at the lateral and apical angles.
Cell dimensions: L: 48-53 µm; B: 38-47 µm; Is: 13.8-16 µm; Sp: 8-18 µm; Overall LxB: 59-82 x 53-72 µm.
West & West (1912: 60) only record it from Sutherland, Scotland and the Outer Hebrides (specifically in the plankton of lochs on the Isle of Lewis). It would appear to have an Atlantic distribution, but rarely encountered. The only recent records from the Outer Hebrides came from squeezed Sphagnum from a bog and from the edge of a stream on Lewis.

References: 
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.