Vegetative hyphae: hyaline, hardly discernible in thallus sections Pseudothecia: black, globose to obpyriform, sessile, 100-140 µm high and 120-180 µm wide wall: consisting of 2 layers, the outer layer dark brown, entirely carbonized, inner layer basally hyaline to pale brown hamathecium: with pseudoparaphyses of type a asci: clavate to saccate, 40-60 x 12-15 µm, 8-spored, with ocular chamber; endoascus: I -and KI -, BCr- ascospores: hyaline, 1-septate, not constricted at the septa, upper cell slightly thicker than lower one, ellipsoid , 11-17 x 5-7 µm; walls: smooth, thin, BCr- Pycnidia: not seen. Hosts: thallus areolae or sometimes on apothecial discs of Acarospora subg. Xanthothallia, incl. A. heufleriana, A. lavicola, A. radicata, A. schleicheri and A. xanthophana World distribution: Africa, Australia, and North and South America Sonoran distribution: known from many localities in Arizona and southern California, Baja California, and Baja California Sur, Chihuahua, Sinaloa and Sonora. Notes: There are at least three species of Stigmidium s. l. occurring on Acarospora: S. epixanthum, S. fuscatae (see below) and S. rouxianum Calat. & Triebel (see Calatayud and Triebel ). The ascomata of S. epixanthum are mostly sessile, are often found scattered or in irregularly small groups, preferring the edges and fissures of the yellow thallus areoles.