Thallus: crustose, thin, continuous or rimose, plane or minutely rugose surface: light or dark gray, dull; margin: usually indeterminate; prothallus: lacking; vegetative propagules: absent Apothecia: sessile, frequent, often contiguous, sometimes angular by compression, up to 0.3-0.7 mm in diam. disc: black, plane, occasionally becoming convex thalline margin: concolorous with thallus, 0.05-0.1 mm wide, entire, persistent or sometimes excluded; excipular ring: absent thalline exciple: 45-70 µm wide laterally; cortex: 5-10 µm wide; cells: up to 4.5-5 µm wide, rarely pigmented; algal cells: up to 9-15.5(-17) µm in diam.; thalline exciple: 60-80 µm wide below; cortex: 10-20(-30) µm thick, hyphae intricate proper exciple: hyaline, (5-)10-15 µm wide, expanding to 10-20 µm at periphery hymenium: 80-100 µm tall; paraphyses: 2-2.5 µm wide, often conglutinate, with apices up to 4-5 µm, heavily pigmented, forming a dark brown or reddish brown epihymenium; hypothecium: hyaline, 50-80 µm thick asci: clavate, 60-70 x 15-20 µm, 8-spored ascospores: brown, 1-septate, broadly ellipsoid, type A development, Dirinaria-type, (15.5-)17.5-18.5(-20.5) x (8.5-)9.5-10.5(-12) µm, some spores dilated at septum when young, more so in K, lumina angular, Physcia-like at first, becoming rounded, usually retaining thick apical walls but sometimes almost filling the cells; finally developing pigmented endospore wall in overmature spores; torus: narrow, only in mature spores (best seen in K); walls: lightly ornamented Pycnidia: immersed in thallus, ostioles becoming darkly pigmented; conidiophores: type-V conidia: bacilliform, 3-4 x c. 1 µm Spot tests: K+ yellow, C-, KC-, P- or P+ faint yellow Secondary metabolites: atranorin in cortex. Substrate and ecology: on deciduous trees, most frequently on twigs, also on wood; frequently collected with R. santae-monicae but also with R. badiexcipula, R. capensis, R. laevigata, and R. marysvillensis World distribution: a North American endemic belonging to the Californian floristic element along the coastal ranges, north to Saltspring Island, British Columbia, south to Baja California, rare in the Sierra Nevada where it occurs to 1340 m, its highest recorded elevation Sonoran distribution: southern California, and south to El Rosario, Baja California. Notes: Rinodina californiensis typically does not become areolate, but nevertheless is similar in external appearance to a diminutive R. capensis, that has larger, Physcia-type spores with more angular lumina, and more persistently thickened apical walls, in addition to the massive, columnar apothecial lower cortex. Rinodina santa-monicae is distinguished by lacking atranorin, possessing a dark brown, heavily pigmented epihymenium, a frequently prominent excipular ring, and an incompletely formed thalline margin. Rinodina marysvillensis might be mistaken for R. californiensis in the absence of disc pruina but is distinguished by its more prominent apothecial margin, columnar lower cortex, and presence of pannarin in the epihymenium.