Thallus: crustose, thin, continuous becoming rimose, rarely areolate, areoles up to c. 0.8 mm wide, plane surface: gray or more usually ochraceous to pale brown, dull; margin: determinate; prothallus: thin, entire, dark brown; vegetative propagules: absent Apothecia: adnate, frequent, rarely contiguous, up to (0.35-)0.7-0.9 mm in diam. disc: dark brown to black, typically becoming pruinose, plane at first, becoming convex thalline margin: lacking; proper margin: biatorine, concolorous with disc, 0.05-0.1 mm wide, entire, sometimes becoming excluded thalline exciple: lacking proper exciple: (50-)70-100 µm wide, pigmented light brown in part; algal cells: occasionally found in base of exciple hymenium: 80-110(-140) µm tall; paraphyses: 1.5-2 µm wide, conglutinate, with apices up to 2.5-3 µm wide, hardly pigmented, immersed in dispersed pigment, forming a red-brown epihymenium, crystals on surface; hypothecium: hyaline, 65-105 µm thick asci: clavate, 60-80 x 17-24 µm, 8-spored ascospores: brown, 1-septate, narrowly ellipsoid, type A development; Physcia-type, (17-)20.5-22(-25) x (8-)9.5-10.5(-12) µm, lumina angular at first, quickly becoming rounded, apical walls mostly remaining thick; torus: narrow, well developed in some spores; walls: ornamented Pycnidia: not seen Spot tests: all negative Secondary metabolites: zeorin, ± variolaric acid. Substrate and ecology: mostly on deciduous trees (Quercus agrifolia and Q. tomentella) but also on conifers World distribution: a North American endemic with an oceanic distribution from the Queen Charlotte Islands, British Columbia, to southern California Sonoran distribution: Santa Cruz Island, San Clemente Island and San Gabriel Mountains, at elevations of 395-1675 m. Notes: The thin, continuous to rimose, ochraceous or brownish thallus, and the large, biatorine apothecia with pruinose discs make R. hallii distinct among North American Rinodina species on bark. It may be related to R. trevisanii that also has a similar thallus morphology and chemistry, and also develops biatorine apothecia. Rinodina trevisanii is distinguished by its mostly smaller apothecia, absence of pruinose discs, and significantly smaller spores belonging to the Physconia-type. There are no recent mainland records from the study area.