Opegrapha longissima: Müll. Arg., Bull. Herb. Boissier 3(2): 43 (1895). Life habit: lichenized Thallus: thin, smooth to warted, continuous to rimose surface: whitish, pale grey, greenish-grey or pale brownish, matt Ascomata: lirelliform, flexuous, immersed to slightly sessile, (0.4-)1-4(-6.5) x 0.15-0.25 mm, often branched, scattered or contiguous, sometimes stellate; disc a slit, soon widely exposed, reddish brown, epruinose or sometimes more or less white pruinose exciple: widely open below the hymenium, dark brown, K± olivaceous, 15-55 µm wide hymenium: 65-80 µm tall, I+ reddish; epihymenium, dark brown, K± olivaceous; hypothecium pale brown, 15-30 µm, K± olivaceous; paraphysoids richly branched, anastomosed, 1-1.5 µm wide asci: clavate, 8-spored, 50-70 x 12-15 µm (between Variatype and Vulgata-type) ascospores: oblong to fusiform, sometimes clavate, hyaline, becoming dark brown granulose when over-mature, (16-)18-22(-24) x (3-)3.5-4.5(-5) µm, (3-)4-7-septate, brown pigmentation probably due to a change in the perispore (Parasitica-type) Pycnidia: not observed Spot tests: thallus P-, K-, KC-, C-, UV- Secondary metabolites: none detected. Substrate and ecology: epiphytic, on Jatropha in Baja California Sur World distribution: North America (SE of U.S.A., Mexico) Sonoran distribution: Baja California Sur (one locality S of Todos Santos). Notes: This species was known so far only from SE of the U.S.A. and is reported here for the first time from Mexico. It is characterized by the exciple that is widely open under the hymenium, and long and flexuous lirellae with a widely exposed, reddish brown hymenial disc. In the only specimen from the Sonoran Desert Region (van den Boom 24798), the mature ascospores are mainly 5-septate.