Published online by Cambridge University Press: 18 January 2017
A new Arthothelium species, A. hymeniicola in the apothecia of an unidentified Bacidia sp., is described from Campbell Island, New Zealand. The new species is remarkable, not only by being the first lichenicolous Arthothelium species, but also by being an endohymenial fungus not forming ascomatal structures. Previously, no Arthoniaceae species were reported as parasitic from Bacidia spp. However, both generic and family placements are tentative and the possibility of a placement in the Cookellaceae is also discussed. Differences between Arthoniaceae and Cookellaceae, and the generic delimitations within them, are generally based on the structures of the fruiting bodies, rendering the taxonomic placement of the new species challenging without molecular data; neither fruiting bodies nor molecular data are available for the new species and the Cookellaceae.