Thallus: fissured or rimose to rimose-areolate, with thin to moderately thick verrucae; margins: entire, unzoned, prothallus white or lacking upper surface: whitish gray to créme colored, rough, dull, whitish gray pruinose; lacking isidia and soredia fertile verrucae: concolorous with thallus, pertusariate, numerous, c. 0.5-1.3 mm in diam Apothecia: 1 to 3(-5) per verruca, semi-immersed to plane, sessile, roundish, apothecial margin entire, verrucose, crenulate to fissured, often showing a split between apothecial margin and verrucae disc: dark brown to black, epruinose or grayish pruinose; epithecium: hyaline to brownish or blackish, K+ weakly violet; hypothecium: hyaline to yellowish or reddish asci: cylindrical, 100-200 x 15-28 µm, 8-spored ascospores: uniseriate to rarely biseriate, single-celled, hyaline to brown or blackish brown, K+ violet, globose to subglobose, 12-30 x 12-20 µm; spore walls: bilayered, evenly thickened, 3-7µm; outer spore wall: 2-5µm; inner spore wall: 1-2µm Pycnidia: not observed Spot test: K-, C+ orange; KC+ orange, P-, UV+ orange to weakly orange Secondary metabolites: arthothelin (major), 2,4-dichloronorlichexanthone (minor), 2,5-dichloronorlichexanthone (minor), and 4,5-dichloronorlichexanthone (minor), or secondary metabolites lacking. Substrate and ecology: occurring on limestone and sandstone in coastal or subcoastal areas at 20 to 300 m World distribution: endemic to the southwestern coast of North America Sonoran distribution: islands offshore southern California and on mainland in Baja California. Notes: Pertusaria occidentalis is characterized by its pertusariate ascomata, its globose to subglobose, K+ violet ascospores, 8-spored asci, and the presence of the arthothelin chemosyndrome. Morphologically, the species is easily recognized by a split between the verrucae and the apothecial margin. The thalli are gray pruinose, especially prominent in calcicolous specimens. Pertusaria melanospora is similar in having 8-spored asci and containing the arthothelin chemosyndrome. It is, however, readily distinguished by its larger, ellipsoid ascospores (60-80 x 35-47 µm) and its epruinose, yellow to yellow-brown thallus.