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 but initially lichenicolous on Lecanographa Thallus: effuse, granulose or verrucose, poorly delimited, in section up to 1 mm thick, without a prothallus surface: cream-colored, smooth to slightly farinose medulla: differenciated, cretacous, with numerous granules and K+ soluble crystals photobiont: trentepohlioid algae Ascomata: irregularly rounded, 0.4-1 mm wide or slightly elongated and up to 1-1.5 x 0.3-0.8 mm, immersed, isolated disc: brown to reddish brown, with whitish pruina epihymenium: brown, end cells not or slightly thickened hymenium: hyaline, 60-70 µm tall; paraphysoids: branched and anastomosing, up to 1.5 µm thick; subhymenium: hyaline asci: clavate, 45-65 x 20-25 µm, 8-spored ascospores: persistently hyaline (only when old brown and warted), 3-4(-5) septate, upper cell enlargened, oblong ovoid, 14-17 x 6-7 µm, without or poorly developed gelatinous sheath Pycnidia: not observed Chemical reactions: thallus I+ red or blue, K-, C+ red, P-; hymenium I+ blue turning red, KI+ blue Secondary metabolites: none detected by TLC. Substrate and ecology: on rocks World and Sonoran distribution: known only from the type in central California (Monterey Co.). Notes: Strictly speaking, it occurs north of the Sonoran area but may eventually be found farther south. The C+ thallus reaction implies that something like gyrophoric acid or lecanoric acid should be present, although apparently in low concentration. Two other more robust Arthonias occurring on rock (A. gerhardii and A. verrucosa) have C- and P+ yellow (orange) thalli indicative of psoromic acid being present. A fourth species, A. madreana, has a more poorly developed thallus that is both C- and P-.