Lecanoromycetes, Lecanoromycetidae, Lecanorales, Megalariaceae,
Thallus crustose, often thin and inapparent: photobiont green, Chlorococcaceae.
Ascomata apothecia, round, disc exposed with a proper margin.
Proper exciple of radiating hyphae
Hymenium colourless, or streaked with pigment,I+ blue, epihymenium usually strongly pigmented green or magenta.
Hamathecium of paraphyses.
Hypothecium pale- to dark-brown.
Asci cylindrical, Lecanora or Biatora-type; ascospores colourless, 1-septate, ellipsoidal.
Conidiomata rarely present;conidia, short rods.
Megalaria is a genus of c. 20, mainly corticolous species, although many others await description or transfer from Catillara. They are known primarily from tropical and temperate regions of both hemispheres, although one species occurs in the Arctic and several are confined to the Southern Subpolar Region.
The genus was described to accommodate M. grossa by Hafellner (1984) and included in its own family, the Megalariaceae. Subsequently, several Northern Hemisphere species were added and Ekman & Tønsberg (1996) reviewed the genus and concluded that, although M. grossa differed from the other species in several characters, the genus was best circumscribed in a wide sense emphasizing the similarities between them rather than the differences. Fryday (2004a) has described two new saxicolous species from Campbell Island (New Zeland) and a muscicolous species from Kerguelen, but several other species are known, either described in Catillaria or apparently undescribed.
Recently, Kalb (2007) removed the species with a two-layered exciple and a thallus containing zeorin (including M. melanotropa) into the new genus Catillochroma Kalb. However, as species with an intermediate exciple-type occur, and these do not correlate wth the presence/absence of zeorin, I prefer to retain all the species in Megalaria pending a thorough revision of the genus.