Thallus: crustose, thin, rimose at first, usually becoming areolate, areolae up to c. 0.4 mm wide, plane to rugose, margins often raised and swollen surface: light or dark gray, to greenish-brown, dull; margin: indeterminate; prothallus: lacking consoredia: rarely developing from areole margins, 20-40 µm in diam., or blastidiate blastidia: 100-120 x c. 200 µm, breaking into consoredia Apothecia: erumpent at first, remaining adnate, up to 0.5-0.6 mm in diam. disc: black, often concave at first, becoming plane, only the largest becoming convex thalline margin: concolorous with thallus, persistent, 0.05-0.1 mm wide, sometimes undeveloped or only partially developed; excipular ring: typically present, confluent thalline exciple: hyaline, 50-80 µm wide; cortex: 10-20 µm wide; epinecral layer: sometimes present, 5-10 µm wide; cortical cells: up to 4.5-6 µm wide, usually pigmented; algal cells: up to 8-12.5 µm in diam.; thalline exciple: 45-90 µm below; cortex: 10-20 µm proper exciple: 10-15 µm wide laterally, expanding to 20-30(-50) µm at periphery, 40-70 µm wide laterally and pigmented when thalline exciple absent hymenium: 80-100 µm tall; paraphyses: 2-2.5 µm wide, somewhat conglutinate, with apices up to 4.5-5 µm wide, heavily pigmented, forming a dark brown epihymenium; hypothecium: hyaline, 60-80 µm thick asci: clavate, 5570 x 16-20 µm, 8-spored ascospores: brown, 1-septate, ellipsoid, type A or B development, Dirinaria-type, (14.5)17.5-19(-22) x (7-)9-10(-11.5) µm, young spores inflated at septum, more so in K, sometimes curved and elongately ellipsoid, becoming more broadly ellipsoid, lumina angular (Physcia-like) at first, becoming rounded, mostly retaining thickened apical walls at maturity, endospore wall becoming pigmented in overmature spores; torus: absent; walls: darkly pigmented, lightly ornamented Pycnidia: immersed, ostioles brown; conidiophores: type I conidia: bacilliform, minutely stalked, 2.5-3 x c. 1 µm Spot tests: all negative, Secondary metabolites: none detected. Substrate and ecology: on bark, mostly collected on twigs of deciduous species but also on Pinus sp., at elevations of 15-1525 m, with R. bolanderi, R. californiensis, and R. herrei World distribution: a North American endemic belonging to the Californian floristic element with an oceanic distribution (Washington to southern California) Sonoran distribution: frequent in coastal, southern California. Notes: Rinodina santae-monicae is closely related to R. californiensis by spore type and the two species frequently grow together. It is distinguished by a consistent lack of atranorin, dark brown epihymenium, the frequent presence of a confluent excipular ring and/or poor development of the thalline margin, and the development of the areoles that may possess swollen and somewhat elevated margins that may become consorediate or blastidiate. Two other species of coastal southern California, R. herrei and R. poeltiana, may possess blastidia. The blastidia of R. her-rei also develop from the areole margins but this species is readily distinguished by its brown thallus, convex apothecial discs, and larger spores. Rinodina poeltiana has more abundant blastidia developing over the whole surface of the areoles. This species is also distinguished by its slightly shorter, Pachysporaria-type spores.