Thallus: endolithic disc: blackish red to black (red when wet), plane, epruinose margin: carbonized, slow to hydrate, thick, prominent, verrucose, wavy, early becoming crenulate and warty, splitting into sections, sometimes elongating and covering disc and opening when wet true exciple: up to 70 (-80) µm thick laterally and underneath medulla, slowly hydrating; externally black and carbonized, internally sharply delimited, dark to pale yellowish red epihymenium: yellow-brown, not carbonaceous, 10-15 µm thick hymenium: hyaline, conglutinated with yellowish gelatinous substance, 85-125 µm tall; paraphyses: in a yellowish gelatinous gel above, basally 1.7-2 µm wide, apically expanded to ±3.5 µm hypothecium: above brown in thick sections, indistinct from subhymenium and upper medulla, 60-100 µm thick, below white with some reddish hues, narrowing to attachment, up to 0.5 mm thick, incased in exciple, obscured by calcium oxalate crystals asci: clavate, 65-80 x (7 Apothecia: 12-15 µm, 100+-spored; dispersed or in small groups, (0.3-)0.7-1 mm thick, 1-3(-6) mm wide, narrowly sessile to substipitate cospores: hyaline, simple, narrowly ellipsoid, 4-6 x 1-1.5 µm Pycnidia: not observed in Sonoran material; elsewhere in wart-like, multilocular verrucae; conidia: ellipsoid, 2-3.5 x 1-1.5 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on vertical surfaces of hard acid rock in moist places along rivers and streams, above pools, or rocks periodically flushed or inundated with water, often in canyons or around waterfalls World distribution: Europe, Greenland, North America, and Africa Sonoran distribution: Arizona and southern California. Notes: Sarcogyne clavus is unique among species of Sarcogyne with its narrow point of attachment, its black hypothecium, its carbonized outer exciple encasing a thick medullary area beneath the hypothecium, and its warty, crenulate, or split margin, in-folding when dry in well-developed specimens. As with other epruinose species, rare specimens with pruinose-looking discs are caused by flooding. As with S. similis, which is also usually found on acidic rocks, S. clavus rarely can be found on carbonaceous rocks where the surface is flushed with more acidic water.