Life habit: lichenized Thallus: crustose, epilithic, effuse, rimose to areolate, with a thin black prothallus; in section up to 100 µm thick, ecorticate and without a distinct medulla, I+ light blue in part surface: smooth, creamy white to grayish Ascomata: 0.3-1.3 x 0.2-0.3 mm, simple or slightly branched, numerous, subimmersed to adnate, scattered or aggregated, with a slit-line disc, becoming slightly widened when old, epruinose or, rarely, white-pruinose exciple: closed, 30-50 µm laterally, 70-150 µm at base, K+ olivaceous hymenium: hyaline, 60-80 µm, I+ blue turning quickly reddish; subhymenium: pale brown, 10-15 µm tall, I+ reddish; paraphysoids: to 1.5 µm wide asci: cylindric-clavate, 45-55 x 10-12 µm (Vulgata-type) ascospores: 13-17 x (3-)3.5-4.5 µm, (2-)3-septate, ellipsoid to fusiform, straight or slightly curved, without a distinguishable gelatinous sheath (Calcarea-type) Pycnidia: immersed to subimmersed, ± globose conidia: 4-7 x 0.8-1 µm, straight Spot tests: thallus K+ yellowish, C+ reddish, P- Secondary metabolites: with erythrin and lecanoric acid. Substrate and ecology: on ± acidic rocks (e.g., argillaceous rock, basalt, sandstone), in ombrophobous (shielded from rain) communities, often with Dirina paradoxa, Graphis saxorum, Roccella babingtonii, R. fimbriata, Hubbsia parishii, and Sclerophyton spp. World and Sonoran distribution: coastal, southern California and Baja California. Notes: The C+ reddish thallus and 3-septate ascospores of O. brattiae may suggest affinities to O. gyrocarpa Flot. However, in this latter species the ascomata are gyrose, the ascospores are bigger (17-30 x 3-6 µm), and the thallus is sorediose. Most specimens named as O. calcarea Sm. & Sowerby from California probably belong to O. brattiae. In O. calcarea, the thallus is C-, the asci are wider [(14-)15-18(-20) µm] with an apical nasse, and the ascospores are slightly larger (15-18 x 4-6 µm).