Thallus: epilithic, determinate, verrucose-areolate, c. 0.7-0.8 mm thick, impregnated with rock fragments and crystals, upper surface: pale brown with an olive-green tinge, dull, ecorticate, K-, prothallus not apparent, basal layer absent; algae: Trentepohlia; cells (12-) 14.6-17.1-19.6 (-23) x (11-) 12.6-14.3-16.0 (-19) µm [n = 42] Perithecia: very numerous, hemispherical to subglobose, not attenuate at the base, half-immersed in the thallus, solitary or more rarely crowded in small groups of 2-3 (Fig. 81), 0.38-0,45-0.52 (-0.63) mm in diam. [n=30]; apex: rounded or slightly flattened; ostiole: often slightly concave and quite large, with periphyses up to 15 µm long involucrellum: contiguous with the exciple and extending to excipular basal level, often continuous, opaque, in section dull brown to blackish, K-, the upper parts H2SO4 golden brown, then red after some minutes, up to 60 µm near the apex, covered by a thin hyaline layer up to c. 7-12 µm thick, better seen after treatment with H2SO4; exciple: colorless, up to 30-40 µm near the ostiole, and up to 60 µm near the base; hymenium: inspersed with minute granules; subhymenium: 25-30 µm thick hamathecium: composed of simple, filiform paraphyses, less than 1 µm in thickness, not constricted at septa, obscurely branched at the tips asci: 135-160 x 10-13 µm, with a rounded apex and an indistinct apical chitinoid ring in congo red, I-, cytoplasm I+ yellow, 8-spored ascospores: colorless, 7 (-9) -septate, narrowly fusiform or elongate-fusiform, straight, with acute ends, (31-) 40-46-52 (-62) x (3.5-) 4-6 µm, length/width ratio (6.5-) 8.1-9.8-11.5 (-15.5) [n = 99, 10 perithecia examined]; walls: some immature spores with a thin, gelatinous perispore Pycnidia: rare, immersed or semi-immersed, blackish conidia: 2-4 x c. 0.7 µm Spot tests: all negative, UV negative Secondary metabolites: none detected. Substrate and ecology: on a seepage track of north-facing exposed conglomerate rocks effervescent in HCl, not overgrowing the largest siliceous pebbles present on the surface of the substratum (see Fig. x) World and Sonoran distribution: southern California, currently only collected at a single locality on Santa Rosa in the Channel Islands. Notes: The epithet peregrina means pilgrim, and was selected because the type material, collected in California, was sent from Italy to several fellow lichenologists all around the world. This very distinctive lichen is characterized by a thick thallus, moderately large 7 (-9)-septate ascospores, the presence of periphyses, and the size of conidia. The reaction of the involucrellum to K and H2SO4 distinguishes it from species of sect. Limosagedia, although in P. peregrina some portions of the involucrellum near the ostiole may occasionally react K+ blue. In the keys to the saxicolous taxa of Porina of McCarthy (2000) the new species would key out close to P. riparia P. M. McCarthy and P. curnowii A.L. Sm. However, P. riparia has a thinner, smoother thallus, smaller perithecia, discontinuously smaller asci and 4-10 x 1.5 2.2 µm conidia, while P. curnowii has a very thin thallus, perithecia lacking periphyses and shorter, narrower and persistently 7-septate ascospores.