Thallus: epilithic, continuous, rimose or rimose-areolate, (1.3-)4-6(-8.5) cm in diam. surface: pale greenish gray to dark grayish olive, or yellowish to brownish gray, sometimes shiny photobiont: trebouxioid Apothecia: round to subangular, rarely irregular; occasionally confluent disc: mostly light gray to yellowish white, or light grayish to yellowish brown, rarely light to deep orange, often with a proper exciple forming a thin whitish ring around disc, concave to almost flat, (48-)100-220(-335) µm in diam. thalline margin: indistinct to slightly prominent exciple: lateral proper exciple hyaline, (8-)35-45(-100) µm thick, K-, N-, texture very variable, rarely with granules (epipsamma) epihymenium: hyaline, N-, K- hymenium: hyaline, (57.5-)70-105(-127.5) µm tall, I-, N-, K- paraphyses: straight, branched in upper part, weakly anastomosed, cells slightly constricted to moniliform, larger at apex subhymenium: hyaline, (12.5-)20-28(-42.5) µm thick, I+ blue or I-, N-, K- hypothecium: hyaline, (0-)8-44(-87.5) µm thick asci: 8-spored, tholus I- ascospores: hyaline, simple, ellipsoid, (9.5-)11-14(-16.5) x (3-)5.2-6.4(-8) µm, ±halonate Pycnidia: 25-70 µm in diam., hyaline to tan, immersed to prominent conidia: bacilliform, 2.5-3.9 x 1 µm Spot tests: thallus K-, C-, P- Secondary metabolites: none detected. Substrate and ecology: on dry siliceous boulders in deciduous forests World Distribution: North America, primarily in the Appalachian-Great Lakes region Sonoran distribution: southern California (Santa Monica Mountains). Notes: Ionaspis alba is characterized by a light to dark olive-gray, rarely brown-gray color, and the apothecia are typically pale gray, and usually lack an epipsamma. These characters distinguish the species from Ionaspis lacustris. The thallus color of I. alba might resemble Ionaspis odora, but the negative K-reaction of the hymenium and the presence of trebouoxoid algae in I. alba separate these two species. See Lutzoni (1994) for a more detailed description. The collection from Santa Monica Mountains was originally determined by Zahlbruckner as Lecanora lacustris according to a note on the label by Hasse. The collection is very scanty and is composed of 4-5 small, very thin specimens, up to 1 cm in diam., with olive-gray to olive-brown or gray-brown thallus which is slightly shiny. The apothecia are pale gray to gray-brown, the hymenium is 65-87 µm tall and K-, N-, the paraphyses slightly broader at apex, the spores measures 12-15 x 5-7 µm, and the algae are trebouxioid. It grows on acidic rocks, but the habitat is not further specified on the label.