Thallus: crustose, continuous or verrucose-areolate; prothallus: not visible areoles: flat or verruculose, thin, opaque, ecorticate surface: yellowish white to yellowish gray or whitish gray to gray, smooth or rough, epruinose, with an indistinct margin, esorediate Apothecia: sessile or adnate or constricted at the base to almost subpedicilate, 0.5-1.5 mm in diam., lecanorine disc: brown or orange-brown, plane or convex, epruinose or slightly whitish gray pruinose margin: concolorous with thallus, thick, persistent, prominent, entire or flexuose, smooth, entire or verruculose, without a parathecial ring amphithecium: present, with numerous algal cells, with large crystals insoluble in K, corticate; cortex: hyaline or pale yellow, distinct, basally thickened, gelatinous or interspersed, (20-)25-30(35) µm thick laterally, (30-)55-75(-80) µm thick basally parathecium: hyaline, lacking crystals epihymenium: red-brown to orange-brown, with pigment dissolving in K, with crystals dissolving in K hymenium: hyaline, clear; paraphyses: slightly thickened (up to 3 µm wide) apically, not pigmented; subhymenium: hyaline, 15-20 µm thick; hypothecium: hyaline, without oil droplets asci: clavate, 8-spored ascospores: hyaline, simple, broadly ellipsoid, (10-)10.5-14(-17) x (7-)9-11(-11.5) µm; wall: more than 1 µm thick Pycnidia: not seen Spot tests: K+ yellow, C-, KC-, P- or P+ pale yellow Secondary metabolites: atranorin (major), chloroatranorin (minor), gangaleoidin (traces or absent), nephrosteranic acid (minor or absent), roccellic acid (major), in a few samples usnic acid in traces. Substrate and ecology: on bark of deciduous trees, but also on some conifers World distribution: endemic to western North America Sonoran distribution: Baja California and southern California. Notes: Lecanora pacifica is a very variable species and thus easily confused with other species of the genus. In its common form, the species is readily distinguished by a very thin thallus, large, flat apothecia, with disc varying from from yellow to olive-brown or black, but usually with a gray tinge. The disc are often slightly pruinose. However, morphotypes with small apothecia may be confused with L. chlarotera or "Straminella" spp. However, these species are usually distinguished by their differing chemistry. L. demosthenesii may also be similar; the differences are discussed under that species.