Flora of New Zealand Series

Thallus crustose, uniform, membranaceous, mucilaginous, in a thin film, sometimes developed within substrate, effuse or lacking. Photobiont green, Trebouxia. Ascocarps perithecia, simple, immersed or superficial, exciple carbonaceous or horny, dark or pale, opening to exterior by a poriform ostiole. Paraphyses persistent, slender, unbranched. Asci 4-8-spored. Ascospores ellipsoid, simple, colourless.

Thrombium is distinguished by having a perithecium for ascocarp, persistent unbranched paraphyses, and simple, colourless spores. The persistent paraphyses separate it from Verrucaria, in which these structures become swollen and gelatinised. Their nature is readily recognised when squash preparations are made in Melzer's reagent [Swinscow Lichenologist 2: 276 (1964)]. The genus is doubtfully monotypic and is placed in the family Microglaenaceae. The recent discovery of T. epigaeum in New Zealand is the first record of the genus from the Southern Hemisphere.