Thallus: crustose, areolate, up to 15 mm in diam., up to 0.3 mm thick; prothallus: absent areoles: rounded, convex, 0.2-0.5(-0.9) mm in diam. surface: dark brown, smooth cortex: brown pigmented above, 10-30 µm thick, with an epinecral layer 2-10 µm thick; hyphae: 4-5 µm in diam.; algal layer: 65-100 µm thick medulla: hyphae I- Apothecia: orbicular to irregular in outline, sessile, not constricted below, 0.2-0.4(-0.5) mm in diam. disc: black, plane, dull, epruinose exciple: black-brown peripherally, hyaline to brown internally, 20-40 µm thick above, 50-70 µm thick below, with hyphae 4.5-5 µm in diam. epihymenium: olivaceous to brown, 10-15 µm thick hymenium: hyaline, 50-70 µm tall, I+ red; paraphyses: branched and often anastomosing, c. 2 µm wide below, c. 3.5 µm wide apically; subhymenium: hyaline to brown, 30-45 µm thick hypothecium: dark brown to black-brown, 100-230 µm thick asci: clavate, 35-55 x 11-15 µm, 8-spored ascospores: hyaline to rarely pale brown, ellipsoid, 8-14.5 x (4-)4.5-6(-7) µm Pycnidia: immersed conidia: cylindrical, c. 4 x 1 µm Spot tests: cortex and medulla K-, C+ red, KC+ red, P- Secondary metabolites: gyrophoric acid. Substrate and ecology: on Lecanora rupicola agg. over non-calciferous rock World distribution: bipolar and temperate regions of Europe, North and South America and Australasia Sonoran distribution: central and southern Arizona, southern California and Baja California. Notes: Rimularia insularis is well characterized by its brown thallus, black apothecia and occurrence on Lecanora rupicola aggregate.