Phylum: Cynaobacteria   Family: Rivulariaceae

Calothrix parietina G. Thuret ex É. Bornet & C. Flahault 1886

First described by Thuret in Bornet & Flahault (1886: 366) but, strangely, not illustrated. One of several species of Calothrix, many are mat-forming. This one is unbranched and has a single, basal, heterocyst.
Trichome dimensions: up to 1 mm long; B: 10-14 µm.
Mat-forming on firm surfaces. Probably fairly common.

References: 
John, D.M., Whitton, B.A. & Brook, A.J. (2011 second edition). The Freshwater Algal Flora of the British Isles.
Thuret in Bornet, É. & Flahault, C. (1886). Revision des Nostocacées hétérocystées contenues dans les principaux herbiers de France.