Thallus: continuous to fissured, with thin to moderately thick verrucae; margins: entire, zoned or unzoned upper surface: whitish gray to yellowish gray, smooth to tuberculate, shiny, epruinose; lacking soredia or isidia fertile verrucae: concolorous with thallus, ampliariate to erect, few or numerous, c. 0.6-3.6 mm in diam.; ostioles 1-7 per verruca, hyaline, level to sunken Apothecia: 1-5 per verruca; epithecium hyaline to black, K-; hypothecium: hyaline asci: cylindrical, 250-460 x 35-60 µm, mostly 4-spored ascospores: hyaline, ellipsoid to fusiform, 75-150 x 30-48 µm; spore wall: 2-layered; outer spore wall: c. 1-10 µm thick; inner spore wall: 2-12 µm thick, rough; apices: up to 20 µm thick Pycnidia: not seen Spot tests: K- or + yellow, C-, KC-, P+ yellow to orange, UV- or + weakly orange-pink Secondary metabolites: 4,5-dichlorolichexanthone and stictic acid (both major), constictic, cryptostictic, menegazziaic and norstictic acids (all minor). Substrate and ecology: on bark of Quercus World distribution: North and South America Sonoran distribution: mountainous areas of Baja California Sur and Sinaloa at 800-1900 m. Notes: This taxon is characterized by a gray to yellowish gray thallus with poriform apothecia, mostly 4-spored asci, ascospores with a rough inner wall, and the presence of 4,5-dichlorolichexanthone and the stictic acid complex. A full discussion of the relationships of P. tetrathalamia is given by Dibben (1980). A similar species is P. leioplaca which is best distinguished by the smooth inner wall of the ascospores.