Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2004. Lichen Flora of the Greater Sonoran Desert Region. Vol 2.

Thallus: crustose, continuous or dispersed-areolate; prothallus: not visible, or blackish to bluish brown 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, 0.4-0.8 mm in diam., lecanorine disc: brown or dark brown to blackish brown or gray-brown, plane, epruinose margin: concolorous with thallus, thin or thick, persistent, even, not flexuose, smooth, entire or verrucose or verruculose, without a parathecial ring amphithecium: present, with numerous algal cells, with large crystals insoluble in K, corticate; cortex: hyaline, distinct, basally thickened, gelatinous or interspersed, (20-)30-40(-45) µm thick laterally, 35-80 µm thick basally parathecium: hyaline, lacking crystals epihymenium: brown to dark brown, with pigment dissolving in K, with crystals dissolving in K hymenium: hyaline, clear; paraphyses: slightly thickened apically, not pigmented; subhymenium: hyaline, 15-20 µm thick; hypothecium: hyaline, without oil droplets asci: clavate, 8-spored ascospores: hyaline, simple, broadly ellipsoid, 11.5-17.5 x (7-)7.5-10(-12) µm; wall: more than 1 µm thick Pycnidia: not seen Spot tests: K+ yellow, C-, KC-, P- or P+ pale yellow Secondary compounds: atranorin, chloroatranorin, and roccellic acid. Substrate and ecology: on bark of conifers, but also some deciduous trees, such as Alnus or Prunus, or wood World distribution: a circumboreal species occurring in Asia, Europe, and North America Sonoran distribution: Arizona, southern California, Baja California, and Chihuahua. Notes: Lecanora circumborealis is characterized by the presence of a thick amphithecial cortex widely expanded at the base, and large and broad spores with thick walls. Sometimes, it can be confused with L. pulicaris, which is separated by a thinner cortex and mostly a different chemistry.