Thallus: crustose, thin to thick, often with a predominantly black prothallus when on smooth pebbles, effuse; occasionally immersed and indistinct surface: gray, when well developed of flat areoles, surface cells blue-black pigmented but no well-formed upper cortex photobiont: chlorococcoid green algae, cells 10-12 µm in diam. (not "micareoid") Apothecia: sessile, 0.2-0.4(-0.7) mm in diam. disc: black, usually flat, epruinose proper exciple: well developed, 0.03-0.05 mm wide, sometimes excluded in old apothecia; internally blue-black at outer rim, pale brown within epihymenium: dull green, K+ green, N+ purple-red hymenium: 35-50 µm tall; paraphyses: sparsely branched and anastomosing, 1.5-2.5 µm wide, not capitate; hypothecium: dark brown, 40-80 µm thick asci: cylindrical, 25-30 x 7-10 µm, 8-spored ascospores: hyaline, simple, narrow-ellipsoid, (6-)7-9(-10) x (2-)3-4(-5) µm Pycnidia: frequent, often abundant, scattered on the prothallus or partly immersed in the thallus, gray to black conidia: simple, cylindrical, 3-5 x 1.5 µm Spot tests: K-, C-, KC-, P- Secondary metabolites: none detected by TLC. Substrate and ecology: usually on stones and pebbles World distribution: widespread in Europe, also reported from North America (mostly eastern and central) and Australia Sonoran distribution: one collection from southern California (Santa Monica Mountains). Notes: Micarea erratica differs from most other species of the genus (and all those reported from the Sonoran Region) in the apothecia having a well-developed, persistent proper exciple.