Life habit: lichenized, lichenicolous, or non-lichenized Thallus: immersed or superficial, often rudimentary but occasionally well developed photobiont: if present, trentepohlioid or coccal green algae Ascomata: rounded, elongate or branched exciple: usually not distinct, often composed of peripheral hyphal structures made of thick-walled and/or pigmented hyphae, often resembling those in the epihymenium, or not well developed at all epihymenium: conspicuous and with distinct hyphae or indistinct hymenium: with densely to widely spaced asci, separated by paraphysoidal hamathecium of anastomosing and branched septate hyphae; subhymenium: indistinct to distinct in color and structure, usually composed of densely arranged, ±short-celled vegetative hyphae and variously shaped ascogenous hyphae asci: with two functional wall layers and an internal apical beak, semi-fissitunicate, ovate to subcylindrical, usually stalked, often with KI+ elongated blue ring, with usually 8, rarely fewer ascospores; asci opening by a slit-like rupture of the outer layer, and stretching endotunica layers, with the internal layers stretched further up ascospores: colorless to brown, transversely septate to muriform, oblong to ovoid-fusiform, with a smooth or verrucose epispore, or lacking an epispore Conidiomata: pycnidial; conidiogenous cells: ±cylindrical, phialidic conidia: bacilliform, filiform, or falcate Geography: cosmopolitan Substrate: on bark, leaves, rock, wood or lichens. Notes: Pending further molecular evidence, Arthonia should be treated as a heterogeneous assemblage of ±well delimited species groups. In contrast to traditional concepts, the characters of ascospore septation are weak indicators of relationships. Therefore several species with muriform ascospores previously assigned to Arthothelium are included here in Arthonia, if other characters, especially those of ascomatal construction, are shared. Arthonia in a narrow sense includes species close to A. radiata, which usually forms symbioses with trentepohlioid algae. While A. radiata seems to be absent from the Sonoran Desert, a number of species share Arthonia s. str.'s brownish to olivacous pigmentation of the epihymenial structures but are usually non-lichenized. These include A. glaucella, A. granosa, A. pinastri, A. pruinosella, A. rhoidis, A. sexlocularis, and A. tetramera (while A. punctiformis and A. galactites seem to be missing), as well as species with muriform spores, which were previously assigned to Arthothelium, e.g. A. albopulverea and A. beccariana. These species seem to be intractably variable and without observation of spore septations and spore sizes, the recognition of these species is practically impossible. Most lichenicolous species that occur on hosts with chlorococcal green algae are phenotypically related to this large complex as well. Other species groups appear to be more distinct. For example, the Arthonia cinnabarina group (incl. A. cinnabarina, A. redingeri, and A. speciosa), of which most representatives are characterized by crystallized reddish pigments (usually becoming violet and dissolving in K), differs in its ascomatal anatomy and ascospore septation characters. The A. pruinata group includes species with pale brown colored fruit bodies, which are often covered by a pruina of crystallized compounds produced by the mycobiont. Three saxicolous species (A. gerhardii, A. madreana, and A. verrucosa) seem to represent another distinct group. The relationships of other species with Arthonia are still speculative due to the unclear significance of a low number of characters: Arthonia lecanactidea or A. sanguinea might have separate, isolated positions in Arthonia. This is also true for Arthonia mirabilis with its unusual pigmentation. Egea and Torrente (1995) suggested exclusion of the distinct soil inhabitant A. glebosa from Arthonia, mainly because of its thick dark structures underneath the hymenium. The biological relationships of lichenicolous Arthonia species with their hosts are diverse, ranging from ±commensalic life forms to rather destructive behavior. Species in the Sonoran regions are regarded as moderately aggressive, especially those that infect the reproductive structures of their hosts (A. clemens, A. lecanorina, A. intexta, A. subfuscicola, and A. varians). These species are not exclusively parasites of their fungal hosts, as their hyphae can extend beyond the fungal structures into the algal layers of their hosts, where they may establish appressorial contacts with the photobionts. Many specimens from the Sonoran Desert were incorrectly identified in the past. Typical cases are indicated in the notes to the species. In the early Hasse literature (e.g. 1895 to 1906) the following arthonioid names appear, although they were not subsequently used in the flora (Hasse 1913): Arthonia astroidea Ach. (= A. radiata (Ach.) Ach.), A. astroidea var. swartziana Nyl., A. diffusa Nyl., A. dispersa var. cytisi A. Massal., A. galacitites var. depuncta Nyl. (= A. galactites (DC.) Dufour ), A. interveniens Nyl., A. ochrolutea Nyl., A. quintaria Nyl. and A. spadicea Leight. These names were apparently later not reconsidered by Hasse (1913) and probably should be rejected as occurring in southern California, and therefore they are not treated below. Further species occurrences are dubious and were also not included here, including A. adveniens Nyl., A. polygramma Nyl., A. polymorpha Ach., A. subdiffusa Willey, and A. taediosa Nyl. The latter name was often mistakenly used for A. beccariana, but it has significantly larger ascospores. Arthonia caesia (Flotow) Arnold, a conspicuous species with bluish-red pruina on the discs, and lichenized with coccal green algae, was illustrated for California in Brodo (2001), but I have not seen Sonoran material which fits to this species. It should be noted that some further undescribed taxa likely exist in the area, but these have not been considered, as their relationships with other species could not sufficiently be clarified and material was not available. Pycnidia: c. 0.1 mm; wall: olivaceous brown. 2-3 µm thick Chemical reactions: thallus hyphae I-, KI+ blue; ascomatal gels I+ blue becoming ±red, with KI+ blue and elongated ring-structure in tholus. Substrate and ecology: commonly found on dead twigs in Mediterranean regions, anombro-, photo-, thermophilic, pioneer species, not lichen-forming World distribution: Mediterranean Europe, California and Baja California Sonoran distribution: southern Californa (Channel Islands and mainland), Baja California, and Baja California Sur.