Common Name: arched earthstar
Fruiting body round to globose, 1.5-2.5 cm broad, the outer layer typically with embedded dirt and debris; at maturity splitting into 4-6 rays which recurve and elevate the spore sac, the tips remaining attached to a basal cup; expanded fruiting body up to 6 cm broad, and 8 cm tall; rays dark-brown, the surface scaling irregularly in age to expose a light brown under layer; spore sac 1.5-2.5 cm in diameter, rounded to slightly flattened, on a short stalk, dark brown, with a roughened surface, opening via an apical tear or slit.
Spores 3.5-4.5 µm, round, warted; dark-brown in mass.
Solitary to gregarious under conifers. In our area commonly associated with Cupressus macrocarpa (Monterey cypress); fruiting from late summer in areas of fog drip to mid-winter.
Geastrum fornicatum differs from most earthstars in that the rays, rather than lying flat, "stand on their tips," elevating the spore sac. The recurved rays of Geastrum fornicatum are fixed at maturity and should not be confused with the hygroscopic rays seen in Astraeus hygrometricus and A. pteridis which fold and unfold over the spore sac with changes in humidity.