Diagnosis. Thallus flavo-olivaceus, peltatus, usque ad 3.5 mm latus, subtus non tomentosus, filis rhizinis affixus. Thallus in sectione (160)-186-286-(300) µm altus; stratum tomentosum 20-100 µm. Alga ad ordinam Chroococcales (Cyanobacteria). Apothecia immerse, disci fusci, usque ad 1.5 mm lati. Hymenium denique divisum, (75)-83-117-(135) µm altum; subhymenium 23-36-(40) µm altum; hymenium et subhymenium iodo caerulescente, deinde fusco-rufuscentia. Asci 8-spori, 51-74-(80) · 7-12-(13) µm, ascosporae ellipsoideae, eseptatae (7.6)-9-11.6-(12.3) · 4.4-6-(6.5) µm. Pycnidia non visa.
Type: Mexico, Baja California Sur, along road no. 1, 68 km NE of Ciudad Insurgentes, pass through Sierra de la Gigante, thornbush-vegetation (Acacia, Lycium, Prosopis); 25°36'N, 111°20'W; on gravel at east-exposed mountain slope. 400 m a.s.l. Leg.: B. Büdel & C.M. Wetmore, January 8th, 1989, Büdel 20062e (ASU-holotype; isotypes in ASU, HBG and private herbarium B. Büdel).
Etymology: the epithet “necrocorticata” was derived from the greek word “νέκρωσις” (= nekrosis), to become necrotic, and the latin word “cortex”, describing the outer layer (bark) of plant like organisms.
Thallus yellowish olive, peltate, squamulose, often concave, margins regularly down-curved, up to 3.5 mm in diameter, surface often cracked or wrinkled, attached to the substratum by rhizoidal strands. Thallus (160)-186-286-(300) µm tall. Upper cortex with an epinecral layer, 8.6-17.2-(21) µm thick, cortex often yellowish-brown pigmented in the upper part, (55)-59-84-(88) µm tall, consisting of mycobiont hyphae and dead cyanobiont cells. Photobiont layer (44)-50-85-(92) µm tall, hyphae of both layers 2-3 µm thick. Photobiont a unicellular cyanobacterium, cells 4.3-5.8 µm in diameter, probably belonging to the genus Chroococcidiopsis. No specific photobiont-free medulla developed. Lower cortex not always well developed, if present 22-58-(81) µm tall, followed by a dense web of rhizoidal hyphae.
Apothecia urceolate, up to 1.5 mm in diameter, disc reddish-brown to dark-brown. Hymenium and subhymenium IKI+ blue turning red brown. Hymenium (75) 83 – 117 (-135) µm tall, often multi-divided; subhymenium 23 – 36 (-40) µm tall (figs. 8-9). Asci 8-spored, 51-74-(80) · 7-12-(13) µm, spores ellipsoid, rarely fully developed (7.6)-9-11.6-(12.3) · 4.4-6.0-(6.5) µm (figs. 10, 12). Paraphyses 1 µm thick at the base, reaching 2.4 µm in the apical part (fig. 11).
Pycnidia not seen
Distribution: So far only known from the greater Sonoran Desert region in Mexico. Probably confused with Peltula species in other regions.