Dictyonema coppinsii, a new name for the European species known as Dictyonema interruptum(Basidiomycota: Agaricales: Hygrophoraceae), with a validation of its photobiont Rhizonema (Cyanoprokaryota: Nostocales: Rhizonemataceae)

The lichenized basidiomycete known as Dictyonema interruptum is a widely distributed but rare, oceanic, western European species known from the British Isles, the Pyrenees, the Azores, and Madeira. Unfortunately, the name has never been validly published. The species was first described in the cyanobacterial genus Calothrix in 1833, which predates the starting point of heterocystous bacterial nomenclature, 1 January 1886. The epithet was never subsequently included in a validly published binomial and its combination into Dictyonema was therefore invalid as well. There was also controversy about whether the epithet applied to the cyanobacterial photobiont (as originally intended) or to the lichen fungus (as proposed later). Because of the lack of a valid description for this epithet, we have chosen to establish a new name, Dictyonema coppinsii Lucking, Barrie & Genney, for the lichenized fungus at hand, whereas the cyanobacterial photobiont is validated with the genus name Rhizonema Lucking & Barrie, adopting the epithet interruptum for it, and placed in a separate family, Rhizonemataceae Budel & Kauff ex Lucking & Barrie.