Life habit: not lichenized Thallus: effuse, in the uppermost layers of the bark photobiont: absent Ascomata: ± round to branched (lirellate), up to 1.4 mm in diam., immersed to semi-sessile, 40-90 µm thick disc: flat to somewhat convex, without or with inconspicuous pruina epi hymenium: olivaceous brown, 5-15 µm thick, paraphysoidal end cells slightly thickened, with apical pigment caps, 2-2.5 µm thick hymenium: hyaline, 30-50 µm tall; subhymenium: hyaline, 5-20 µm thick asci: clavate to broadly clavate, of Arthonia type, 22-35 x 15-20 µm ascospores: persistently hyaline, 3-septate, narrowly obovate, straight, 10-15 x 4-5 µm, sometimes (when old) with an epispore Pycnidia: common, immersed, 0.06-0.1 mm in diam. conidia: straight, 3-5 x 1-2 µm Chemical reactions: ascomatal gels I+ persistently blue or ±red, KI+ blue; ascus tholus with KI+ bluish ring-structure; epispore I-, KI-. Substrate and ecology: a photophilic pioneer species, found on smooth bark of conifers and young twigs of deciduous shrubs World distribution: widespread in the Mediterranean area in Europe and in North America Sonoran distribution: southern California, Sonora, Sinaloa, Baja California, and Baja California Sur. Notes: Arthonia pinastri is probably not homogeneous, but at present it is best kept in its wider sense. Forms vary in their reactivity with Lugol, in the density of the asci in the hymenium, and ascomatal shapes. Attempts to split smaller entities are hardly possible, owing to apparently transitional forms.