Opegrapha species with muriform ascospores are revised and two species, differing only in the shape of their excipulum, are accepted: O. arabica, a widespread coastal species, and O. varians, known from mainland Yemen and Socotra. The genus Dictyographa, previously distinguished from Opegrapha by its muriform ascospores, is reduced into synonymy with Opegrapha. Dictyographa angolensis and D. sandwicensis are new synonyms of O. arabica; the type of D. cinerea was found to be a mixture of different species of the Hysteriaceae; and D. contortuplicata and D. psyllocarpa are excluded from the Arthoniales. Lectotypes are designated for D. angolensis, D. contortuplicata and D. sandwicensis.