Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2007. Lichen Flora of the Greater Sonoran Desert Region. Vol 3.
Life habit: lichenized Thallus: grayish white or pale gray, areolate to ±verrucose or occasionally bullate, sometimes delimited by a broad irregular dark brown prothalline line areoles: fissured, with smooth or somewhat farinose surface, effuse, in section up to 1.5 mm thick medulla: thick and cretaceous, with numerous granules and (K+ soluble) crystals photobiont: trentepohlioid algae; cells: up to 20 µm in diam., often single and ±round Ascomata: irregularly rounded, 0.6-2.5 mm in diam. (rarely extended to 1-3 x 0.5-1 mm), immersed or subimmersed, isolated or confluent, disc: dark brown or reddish brown, ±convex epihymenium: brown to reddish brown, 10-20 µm thick; apical cells: poorly or not enlarged, cell walls pigmented brown, with whitish pruina hymenium: hyaline, 80-100 µm tall; paraphysoids: 1.5 µm thick; subhymenium: colorless, 50-80 µm thick asci: clavate, 50-65 x 20-25 µm, 8-spored ascospores: initially hyaline but becoming brown, 2-3 septate, upper cell enlarged, oblong ovoid, 15-18 x 6-7 µm, gelatinous sheath poorly developed or missing Pycnidia: immersed, unilocular conidia: curved, 14-20 x 1 µm Chemical reactions: thalus K+ yellow, C-, P+ orange (yellow); thallus hyphae I+ blue; ascomatal gels I+ blue turning red, KI+ blue, no KI ringstructure in tholus observed; epihymeium and subhymenium I+ blue and hymenium remaining red Secondary metabolites: thallus with psoromic and confluentic acids. Substrate and ecology: on vertical rocks, ombrophobous, in the littoral zone Sonoran distribution: known from southern California (Santa Rosa Island) and coastal NW Baja California. Note: For separation from other Arthonias occurring on rock, see notes under A. infectans.