Phylum: Charophyta   Family: Desmidiaceae

Xanthidium robinsonianum W. Archer 1880

A small cell with the sinuses open and the semicells depressed trapezoid to subreniform. The basal part is concave and the lowers sides about straight and supporting three small granules. The upper part of the sides in convergent and retuse. The apices are truncate. Two or three small granules are present at the apical angles. The centre has a small protuberance comprising usually three small granules. West & West (1912: 83) have the sinuses completely wrong, probably the result of a poor description from Archer. They described and illustrated it as closed with a dilate extremity.
Cell dimensions: L: 23-29 µm; B: 23-28 µm; Is: 7-9 µm; Sp: 1.5-3 µm; Overall LxB: 25-30 x 27-33 µm.
Zygospore: minus projections: 26 µm; overall: 47 µm.
First described by Archer (1880: 114, 116), at an exhibition of the Dublin Microscopical Club, where he found it in Counties Armagh and Donegal. Shortly after a Mr. Crowe found it at Glengariffe, and County Kildare, where he also found a zygospore. Most later records, beyond Ireland, are from Shetland and Sutherland. A generally rare desmid that can be locally common in peaty margins of lochs and abandoned peat-cuttings.

References: 
John, D.M. & Williamson, D.B. (2009). A Practical Guide to the Desmids of the West of Ireland.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.