Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2007. Lichen Flora of the Greater Sonoran Desert Region. Vol 3.
Thallus: rather well delimited, verrucose, surface: ±uneven, whitish gray or brownish gray Apothecia: as in M. [verrucosa var.] verrucosa (generic description) cospores: 34-60 x 30-42 µm. Substrate and ecology: on bark of the trunk and branches of broad-leaved, non-resinous trees (often on Quercus; but also on Juniperus deppeana, Pinus edulis, and Pseudotsuga menziesii), mostly on the bases of ancient trunks; most frequent in the lower montane belt, 700-2860 m World distribution: Europe, Macaronesia; Saudia Arabia, and North America Sonoran distribution: Arizona, southern California, Baja California, and Chihuahua. Notes: Megaspora verrucosa var. mutabilis has different substrates and different altitudinal preferences than M. verrucosa var. verrucosa in western North America. The former's thallus is rather well delimited, being rimose and its surface is whitish to yellowish or brownish gray. Its hymenium is shorter, and its ascospores are somewhat longer and narrower. The thallus of M. verrucosa var. mutabilis also tends to be bare to weakly pruinose and non-scabrous (with granules or crystals mainly in the medulla rather than the epinecral layer), but this is somewhat variable in both varieties.