As part of a comprehensive biodiversity inventory of Galapagos lichens, all species in two closely related genera, Cryptothecia and Herpothallon, are reviewed. Both genera are superficially similar, ecorticate, cottony-byssoid crusts and are unusual insofar as their asci do not develop in distinct ascomata, but instead within ascigerous areas or even solitary inside pseudisidia. Species of Herpothallon typically have an I− medulla and are covered in ecorticate pseudisidia; only a single species is known fertile. Cryptothecia is characterized by ascigerous areas with loosely aggregated asci. In some species the asci are isolated, but others have closely aggregated asci embedded in a hyphal matrix with some carbonization, perhaps indicating preliminary stages towards a development of true ascomata. Lirellate ascomata of the enigmatic, monotypic Helminthocarpon leprevostii show similarity with these ascigerous areas, especially of C. darwiniana and C. galapagoana, two species newly described here. Both also have similar asci and ascospores. As previously suggested, Helminthocarpon should thus not be included in Graphidaceae, but it belongs in Arthoniales, possibly in Arthoniaceae or Opegraphaceae. A key to all species and brief descriptions are provided. Two of the three Cryptothecia species and two of the seven Herpothallon species reported here are new to science. All records apart from Herpothallon rubrocinctum are new to Galapagos and Ecuador.

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