Life habit: lichenized (one species non-lichenized and lichenicolous) Thallus: well developed to immersed and barely apparent (absent in one species). In the M. prasina group the thallus is composed of "goniocysts" resulting in a finely granular structure upper cortex: poorly developed or absent photobiont: primary one a chlorococcoid green algae, cells 4-7 µm in diam. ("micareoid") in all Sonoran species except M. erratica; secondary one cyanobacterium present in a few species (non-Sonoran) Ascomata: apothecial, sessile, often strongly convex, usually immarginate even when young disc: white to pale gray to brown to black proper exciple: poorly developed, usually barely apparent hymenium: paraphyses characteristically thin and branched, often widening toward the apex but never capitate; hypothecium: hyaline to dark brown asci: clavate to cylindrical-clavate, usually "Micarea-type" (KI+ blue apical dome with an apical cushion surrounded by a cylindrical "ring-structure"), 8-spored ascospores: hyaline, simple to multiseptate, ellipsoid to fusiform ta: pycnidial, immersed, sessile or stalked conidia: colorless, simple to multiseptate, bacilliform to thread-like Secondary metabolites: three Sonoran species contain gyrophoric acid (C+ red), one contains methoxymicareic acid; other substances (e.g. argopsin, alectorialic acid, micareic acid, pannarin, prasinic acid, xanthones) present in non-Sonoran species. Geography: a genus of temperate and polar regions in both Northern and Southern Hemispheres, much less frequent in the tropics Substrate: occurring on all non-calcareous substrates, although individual species are generally substrate specific. Notes: The genus is poorly collected in the Sonoran Region, but the true number of species is unlikely to greatly exceed those already reported here. A number of other species have been reported from the Sonoran Region but these collections are either referable to other taxa (i.e. collections of M. elachista and M. lignaria, and are referable to Bacidia s. lat. spp., all records of M. peliocarpa are referable to M. denigrata, records of M. prasina are referable to M. micrococca and Lecania spp., and the single specimen of M. assimilata - collected by Hasse from the Santa Gabriel River - is not a Micarea), or no supporting specimen could be located (i.e. M. hedlundii, which is known from northern California (Humboldt Co.) but not from the Greater Sonoran Desert Region,). However, two of these (M. lignaria and M. peliocarpa) are widespread species that are likely to occur in the Sonoran region and so they are included in the key; good descriptions of these two species can be found in Coppins (1983 or 1992).