Pseudothecia: globose, 150-250 µm in diam., immersed in the host thallus ascomatal wall: colorless in its lower half, intensely green-blue to brown around the ostiole, basally 10-15 µm thick paraphysoids: more or less abundant, 11.5(-2) µm thick asci: cylindrical, 55-75(-80) x 8-11 µm, (4-6-)8-spored ascospores: hyaline, narrowly ellipsoid or fusiform, 1(-2)-septate, (14-)16-24(-30) x (3-)4-6(-7) µm, with the septum centered, strongly heteropolar, with both cells very different in shape and size, with the lower cell curved and attenuated as a tail Pycnidia: globose, c. 50 µm wide conidia: hyaline, simple, bacilliform, c. 3 x 0.5 µm. Hosts: parasymbiotic on different species of Teloschistaceae, mainly on Caloplaca species. In the Sonoran region, on Caloplaca modesta World distribution: C. caudata s. Sonoran distribution: Arizona, Baja California, Baja California Sur, and Sonora. Notes: The different specimens of Cercidospora growing on Caloplaca show an important variability in size and form of the ascospores, and in the relative proportion of the upper and lower cell. The specimens from Sonora growing on C. modesta fit well the original concept of this species, with predominantly 8-spored asci and both ascospore cells having of a similar shape (Kernstock 1895). This author reports Caloplaca lactea as the host of Cercidospora caudata, but, considering the characteristics of the substrate mentioned, on porfidic rocks, the host is most likely another species, C. lactea being a mainly Mediterranean taxon associated with carbonated substrata (Navarro-Rosin's & Hladun 1996). The typical host is possibly C. arenaria, which is more characteristic of the habitats mentioned in the original description, and a frequent host of this fungus. In Europe, C. caudata s. s. is known on Caloplaca arenaria, C. crenularia and C. subpallida.