Thallus: areolate or squamulose, sometimes indeterminate, overall up to 6+ cm wide areoles: round to irregular or lobed, up to 1.3 mm in diam., 0.4-1 mm thick, contiguous; rim: down-turned, sometimes white upper surface: brown to reddish brown to black, sometimes darker toward center of squamules, dull, plane to convex, smooth to warty, usually with abundant creases and fissures, squamules eventually dividing, epruinose latteral cortices: paraplectenchymatous to prosoplectenchymatous, 25-40(-50) µm thick; cells: distinct in water, round to rectangular, ±5 µm wide or to 8 x 4 µm and with ±vertical orientation; syncortex: lacking or up to 15 µm thick; eucortex: upper layer reddish brown to black and 5-9 µm thick, lower layer pale yellow and c. 30 µm thick algal layer: even but becoming uneven, penetrated by hyphal bundles, the bottom of algal layer often becoming feathered into medulla medulla: white, prosoplectenchymatous, indistinct in water attachment: broad, with long white rhizohyphae, branching and anastomosing, often elongating above substrate and gelatinizing, elevating areoles without forming a stipe lower surface: narrow, white and thinly corticated Apothecia: round, 0.1-0.3(-1) mm wide, often lacking, usually one per squamule disc: same color as thallus or darker, epruinose, smooth to rough parathecium: 10-20 µm, expanding to 50-70 µm at surface of apothecium, once observed forming a ring around disc epihymenium: upper 5 µm very dark brown, lower portion reddish brown to a depth of 15 µm hymenium: pale yellow, ±200 µm tall; paraphyses: 1-2 µm wide at base; apices coherent, expanded to 3-4 µm and with black hoods subhymenium: hyaline to pale yellow, indistinct, ±30 µm thick; hypothecium: 10-20 µm thick asci: clavate, 150-170 x 15-25 µm, often only a few per hymenium and under-developed, 100-200-spored ascospores: hyaline, simple, narrowly ellipsoid, c. 1.5 x c. 0.5 µm Pycnidia: not observed Spot tests: UV-, all spot tests negative Secondary metabolites: none detected. Substrate and ecology: pioneer on soil in sunny western exposures, in situations with low competition with other lichens, bryophytes or vascular plants, from coast to high elevations World distribution: western North America (including Washington, Nevada) Sonoran distribution: southern California. Notes: Acarospora terricola was described from a single Hasse collection from Santa Monica Mountains (W!). It may well have a wider distribution than currently known, as it often occurs in infertile patches and could be mistaken as an infertile Placidium or a Psora with its branching, anastomosing rhizohyphae. Apothecia are rare and often immature. The specimen Magnusson determined as A. terricola from Brazil is an undescribed species.