Life form: lichenized, but photobionts sometimes scarcely present Thallus: effuse, in the uppermost bark layers. photobiont: coccal green algae, cells 7-20 µm. Ascomata: rounded, sessile, with constricted base, 0.2-0.5 mm wide disc: black, convex, epruinose and with shiny surface; peripheric hyphal structures: distinct, developed as an excipuloid structure. epihymenium: olivaceous brown, 10-20 µm thick; hyphal tips embedded in brownish ascomatal gel, anticlinally arranged, up to 2 µm wide, without distinct apical caps hymenium: more or less hyaline, 40-60 µm tall; paraphysoids branched and anastomosed, hyphae c. 1.5 µm wide. subhymenium: dark brown, 30-80 µm tall; asci: clavate, 26-35 x 10-15 µm, 8-spored, with thin lateral walls; ascospores: obovate, persistently hyaline, wihtout epispore, 1-septate, incised at septum, the upper cells wider, 10-15 x 3-4 µm; Pycnidia: black, sessile, c. 0.05 mm wide; pycnospores: bacilliform, straight to slightly curved, 3-5 x 1 µm. Chemical reactions: ascomatal gels I+ red, KI+ blue; asci with KI+ blue ring-structure; vegetative hyphae I+ reddish. Substrate and ecology: on smooth bark World distribution: widely distributed in Europe and temperate North America Sonoran distribution: southeastern Arizona (Cochise Co.). Notes: Arthonia patellulata has previously been reported from California, but these records are apparently all incorrect and refer to other (non-lichenized) species. Only the record from Cochise Co. (Nash 18515) clearly matches in its ascomatal characters with A. patellulata, yet, its photobiont is very poorly developed. The type material A. patellulata var. caesiocarpa (Zahlbr.) Hasse represents a poorly developed, non-lichenized species, with 3-septate spores and abundant pruina of bark remnants. It might be a depauperate morph of A. pinastri.