Thallus: foliose, multi-lobed, consisting of many interconnected lobes with numerous holdfasts on the lower side with 4-25 mm wide lobes upper surface: light to dark brown with either type of epinecral layer, turns usually green when wet upper cortex: 60-130 µm thick with the outermost part, c. 10 µm, brownish algal layer: 40-160 µm thick, located in lower part of upper cortex or medulla medulla: 50-240 µm thick, turning red with Melzer's iodine lower cortex: 30-90 µm thick Perithecia: rather common, broadly obpyriform to globose, 210-490 µm high and 150-520 µm, primarily with a hyaline exciple that is light brown to brown in the uppermost part, rarely hyaline throughout asci: clavate or cylindrical, 8-spored ascospores: simple, hyaline, 10.5-20 x 5.5-8.5 µm Pycnidia: common, immersed, with ostiolum similar to that of the perithecium, or located in warts conidia: bacilliform, 4-5 x 1 µm. Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: rocks in seepages or by creeks and lakes World distribution: North America and Europe Sonoran distribution: throughout Arizona. Notes: Dermatocarpon luridum is a common species in seepages and brooks in the temperate zone of both Europe and North America. It is easily recognised by its Melzers reaction, multi-lobed habitus and that it usually turns green when wet.