Thallus: small squamulose to granular, irregularly dispersed over the substrate squamules: semiglobose, up to 1 mm wide upper surface: cinnamon upper cortex: paraplectenchymatous, 30-40 Ám thick medulla: composed of loosely reticulate hyphae cephalodia: dark purplish brown, scarce in Californian material, mostly small, dark pustules beneath the apothecia (or squamules) lower cortex: paraplectenchymatous, 20-30 Ám thick Apothecia: common, up to 2 mm wide with a regular, thalline margin: up to 70 Ám thick, initially smooth, becoming squamulose; disc: cinnamon-colored, often concave; exciple: poorly developed, subparaplechtenchymatous, 20-30 Ám thick; hymenium hyaline but cinnamon above, I+ dark blue, c. 100 Ám high asci: clavate, with a distinct apical, amyloid ring structure, 8-spored ascospores: simple, colorless, ellipsoid, 12-15 x 7-9 Ám, with a distinct rugulose epispore Pycnidia: not observed Spot tests: all negative Secondary metabolites: none detected except for an unidentified pigment. Substrate and ecology: on detritus over soil, in moist, maritime heath World distribution: primarily a cool-temperate to subantarctic species from the Southern Hemisphere Sonoran distribution: only known from Prince Island adjacent San Miguel Island in the Channel Islands of southern California. Notes: It has a similar anatomy to P. tenue, but lacks the secondary products of that species. In addition, P. cinnamomeum has a more thick, urceolate apothecia when young and the cell-lumina of the corticate margin are smaller. The thalli of P. cinnamomeum is usually paler than P. tenue.