Thallus: areolate, rarely rimose at the edge, 1-8 cm in diam., (0.1-)0.2-0.7(-2) mm thick areoles: angular to sometimes round, flat to slightly convex or rarely concave, (0.1-)0.3-0.8(-1.8) mm in diam., contiguous, separated by small cracks prothallus: narrow to rather wide, at the thallus edge, black to gray-black to brown-black or blue-black, rarely with white and black zones (and then white in outermost part), sometimes fimbriate or fringed, 0.1-0.7(-2) mm wide surface: variable, white to gray, brown or ochre, often with a greenish or bluish tinge, often somewhat lighter at the edge of the areoles, sometimes with white lines or spots, dull to ±shiny upper cortex: (15-)20-40(-50) µm thick, uppermost part brown, rarely brown-green, 7-15(-20) µm thick, with cells (4-)5-7 µm in diam.; cortex covered with an epinecral layer 2-15(-35) µm thick, sometimes with ±crystals photobiont: chlorococcoid, cells ±round, 5-16(-20) µm in diam. Apothecia: aspicilioid, often rather numerous in the middle of the thallus, small, 0.1-0.4(-0.8) mm in diam., 1-3(-6) per areole, round to angular or sometimes elongated disc: black, usually with ±white pruina, rarely without pruina, concave to sometimes flat thalline margin: thin, flat to slightly raised, often forming a white or light gray rim around the apothecia, but sometimes concolorous with thallus or rarely darker exciple: I- to partly I+ blue (medially), rarely entirely I+ blue, (20-)30-70(-130) µm; uppermost cells brown, ±globose, (4-)5-6(-7) µm in diam. epihymenium: green to olive to olive-brown, rarely brown, with ±crystals, N+ blue-green to rarely green, K+ brown to green-brown hymenium: hyaline, I+ persistently blue, (130-)140-180(-220) µm tall paraphyses: moniliform to sometimes submoniliform, with (1-)3-6(-8) upper cells ±globose, uppermost cell (3.5-)4-5 µm wide, in lower part 2 µm wide, slightly branched and anastomosing subhymenium and hypothecium: pale, I+ persistently blue, together (30-)40-70(-90) µm thick asci: clavate, 70-120 x (17-)21-35(-45) µm, 8-spored ascospores: hyaline, simple, ellipsoid, rarely subglobose to globose, (14-)19-28(-33) x (10-)12-16(-20) µm Pycnidia: rather common, 1-3(-6) per areole, immersed, often with a white to gray rim, (70-)100-200 µm in diam. (sometimes aggregated in ±branched formations up to 300 µm in diam.), with a black, punctiform to sometimes elongated ostiole, 50-100(-130) µm in diam. conidia: filiform, straight or slightly curved, (8-)9-14(-17) x 0.8-1(-1.5) µm Spot tests: cortex and medulla I-, K+ yellow to red, P+ orange, C- Secondary metabolites: stictic acid as major substance and norstictic acid as minor substance (most common form); rarely only stictic acid (Owe-Larsson 9106; part of Nash 32432, part of Ryan 31112, Tucker 6169-b); or only norstictic acid (part of Ryan 31112; part of Nash 32812; Nash 32825; Wetmore 73150), or norstictic acid as major substance and stictic acid as minor substance (Bratt 11807). Substrate and ecology: on siliceous or volcanic rock (basalt), in both ±exposed and shaded habitats; from close to sea-level on coastal rocks up to 940 m altitude in coastal mountains of California World and Sonoran distribution: central and southern California (from Solano and San Francisco Counties and southwards) and Baja California. Notes: Aspicilia pacifica is a common species on coastal rocks on islands and along the shore of the Pacific Ocean in California and northern Baja California, and it is also found at higher altitudes in the coastal mountains of California. Aspicilia pacifica varies from white to gray to different shades of brown in thallus color, and it is characterized by its rather numerous and small apothecia with ±white pruina and flat to slightly raised thalline margins, its tall hymenium, its large ascospores, its rather short conidia, and the presence of stictic and norstictic acids. Two specimens of A. pacifica have been analyzed by DNA, one from Baja California growing in a coastal shrub community (B. Owe-Larsson 8796) and one from Sandstone Peak in the Santa Monica Mountains, California (B. Owe-Larsson 9109). The DNA studies show that A. pacifica is related to but well separated from A. aurantiaca and A. santamonicae. These species differ also morphologically from A pacifica. Aspicilia aurantiaca has an orange thallus and shorter conidia, while A. santamonicae has a medium to dark brown thallus, scattered and often irregular apothecia usually lacking pruina and has a ±elevated and often prominent thalline margin. All three species contains norstictic acid, but this substance usually appears only in trace amounts in A. pacifica, having stictic acid as a major secondary substance. Aspicilia cinerea differs from A. pacifica by its smaller spores and longer conidia, while A. brucei differs by its smaller spores and shorter conidia.