Thallus: issured or rimose to rimose-areolate, with thin or moderately thick verrucae; margins: entire, unzoned, prothallus lacking upper surface: gray brownish to whitish gray or white, smooth or rough, dull, epruinose or rarely whitish pruinose; lacking isidia and soredia fertile verrucae: concolorous with thallus, pertusariate to pseudolecanorate, numerous, c. 0.5-1.2 mm in diam Apothecia: 1 to 4(-5) per verruca, plane, sessile, roundish, apothecial margin entire disc: blackish brown to black, epruinose; epithecium: hyaline, K+ strongly violet; hypothecium: hyaline to yellowish asci: cylindrical, 180-280 x 27-90 µm, (4-)8-spored ascospores: biseriate, single-celled, hyaline to brown or blackish brown, K+ violet, ellipsoid, (30-)55-80 x 25-50 µm; spore walls: bilayered, evenly thickened, 7-14 µm; outer spore wall: 5-10 µm; inner spore wall: 2-4 µm Pycnidia: not observed Spot test: K-, C-, KC-, P-, UV- Secondary metabolites: 2'-O-methylperlatolic acid (major), confluentic acid (minor), and 2'-O-methylmicrophyllinic acid (minor), or secondary metabolites lacking. Substrate and ecology: occurring on basalt and sandstone in coastal or sub-coastal areas at 5 to 400 m World distribution: endemic to the southwestern coast of North America Sonoran distribution: islands offshore southern California and Baja California, and on mainland in Baja California. Notes: Pertusaria islandica is morphologically variable. Specimens on calciferous sandstone have a thicker, more pruinose, and consequently whitish thallus. Some specimens lack pseudolecanorate discs, but agree in micromorphological and chemical characters with other specimens. The species is easily recognized by its ellipsoid, K+ violet ascospores, its 8-spored asci, its strongly K+ violet epithecium, and the presence of the 2'-O-methylperlatolic acid chemosyndrome. Most thalli are predominantley epruinose. Pertusaria melanospora is similar in having 8-spored asci, but differs in containing the arthothelin chemosyndrome and having a yellow to yellow-brown, shiny thallus.