Twenty-six species recently described from Thailand are revised. Eleven taxa are reduced to synonymy with previously described species, including Leptotrema phaeosporum var. vainiona Räsänen. The following new combinations are proposed: Chapsa calathiformis (Vain.) Lumbsch & Papong, C. laemensis (Homchantara & Coppins) Lumbsch & Papong, Melanotrema melanophthalmum (Homchantara & Coppins) Lumbsch & Papong, Ocellularia albocincta (Hale) Divakar & Mangold, O. guianensis (Sipman) Divakar & Mangold, O. khunantensis (Homchantara & Coppins) Lumbsch & Papong, O. percolumellata (Sipman) Divakar & Mangold, O. subcalvescens (Nyl.) Divakar & Mangold and Ocellularia subgranulosa (Homchantara & Coppins) Lumbsch & Papong.

In the course of an ongoing systematic and taxonomic revision of the lichen family Graphidaceae (including Thelotremataceae), we present world-wide keys to the currently accepted thelotremoid genera and species, excluding the columellate taxa and their relatives of the Ocellularia-Myriotrema-Stegobolus clade (Melanotrema, Myriotrema, Ocellularia, Ocellularia clandestina group, Redingeria, Stegobolus), which will be treated in a forthcoming paper. The keys include all genera and species with chroodiscoid, lepadinoid, and topeliopsidoid apothecia and other taxa featuring periphysoids or fibrils, and their relatives. Taxa keyed out to genus and species level are Acanthotrema, Chapsa, Chroodiscus, Diploschistes, Fibrillithecis, Gyrotrema, Leptotrema, Leucodecton, Melanotopelia, the ‘Ocellularia’ cruentata group, Pseudoramonia, Reimnitzia, Schizotrema, Thelotrema, Topeliopsis and Wirthiotrema. Over 260 species are treated, including a few yet unnamed taxa. The following taxonomic and nomenclatural novelties are introduced: Acanthotrema frischii Lücking sp. nov., Chapsa aggregata (Hale) Sipman & Lücking comb. nov., C. albida (Nyl.) Lücking & Sipman comb. nov.; C. albomaculata (Sipman) Sipman & Lücking comb. nov., C. boninensis (Tat. Matsumoto) Rivas Plata & Mangold comb. nov., C. elabens (Müll. Arg.) Rivas Plata & Mangold comb. nov., C. imperfecta (Hale) Rivas Plata & Lücking comb. nov., C. laceratula (Müll. Arg.) Rivas Plata & Lücking comb. nov., C. magnifica (Berk. & Broome) Rivas Plata & Lücking comb. nov., C. meghalayensis (Patw. & Nagarkar) Lumbsch & Divakar comb. nov., C. meridensis (Kalb & Frisch) Lücking, Lumbsch & Rivas Plata comb. nov., C. mirabilis (Zahlbr.) Lücking comb. nov., C. neei (Hale) Mangold & Lücking comb. nov., C. paralbida (Riddle) Rivas Plata & Lücking comb. nov., C. pseudoexanthismocarpa (Patw. & C. R. Kulk.) Rivas Plata & Lücking comb. nov., C. pulvereodisca (Hale) Rivas Plata & Mangold comb. nov., C. scabiomarginata (Hale) Rivas Plata & Lücking comb. nov., C. waasii (Hale) Sipman & Lücking comb. nov., Fibrillithecis argentea (Müll. Arg.) Rivas Plata & Lücking comb. nov., F. carneodisca (Hale) Rivas Plata & Lücking comb. nov., F. confusa Lücking, Kalb & Rivas Plata spec. nov., F. diminita (Hale) Rivas Plata & Lücking comb. nov., F. eximia (R. C. Harris) Rivas Plata & Lücking comb. nov., F. fissurata (Nagarkar & Hale) Rivas Plata & Lücking comb. nov., F. gibbosa (H. Magn.) Rivas Plata & Lücking comb. nov., Leucodecton desquamescens (Vain.) Lücking comb. nov., L. oxysporum (Redinger) Lücking comb. nov., Schizotrema cryptotrema (Nyl.) Rivas Plata & Mangold comb. nov., Thelotrema patwardhanii (Hale) Rivas Plata & Mangold comb. nov., Topeliopsis guaiquinimae (Sipman) Rivas Plata & Mangold comb. nov., and T. tuberculifera (Vain.) Rivas Plata & Mangold comb. nov. Using the examples of Fibrillithecis halei s. lat., Leucodecton compunctellum s. lat., and Thelotrema monosporum s. lat., we show how difficult species complexes can be flexibly treated in a key, allowing for either a broad concept or the distinction of several individual taxa.

The Thelotremataceae are represented on Aldabra by three species: Leucodecton compunctum, Thelotrema euphorbiae, and Thelotrema hniatukii. The latter is described here as new. It is closely related to T. euphorbiae and know so far only from Aldabra.

Chapsa rubropruinosa Messuti & Codesal collected in Argentina is described as new to science. The species is characterized by a thin thallus, apothecia with a pigmented disc, and transversely septate, hyaline, non-amyloid to faintly amyloid ascospores with thin cell walls. Chapsa minor is recorded for the first time from South America. The new combination Topeliopsis lomatiae (Messuti, Lumbsch & Vězda) Messuti & Mangold is proposed.

The new genus Wirthiotrema Rivas Plata, Kalb & Frisch. is described for the Thelotrema glaucopallens group, based on molecular and morphological evidence, with W. glaucopallens as the type species. The genus is characterized by myriotremoid thalli and apothecia, in combination with a paraplectenchymatous excipulum, I− ascospores, and stictic acid as a secondary compound. The new combinations W. glaucopallens (Nyl.) Rivas Plata & Kalb comb. nov., W. santessonii (Hale) Rivas Plata & Frisch comb. nov., and W. trypaneoides (Nyl.) Rivas Plata & Lücking comb. nov., are proposed and a key to the three species is presented.

Thelotremoid lichens (Ostropales, Ascomycota) with a trentepohlioid photobiont in Tierra del Fuego, the Falkland Islands, the South Island of New Zealand, and subpolar islands of the southern Hemisphere are studied. Twenty-five species are accepted, with three species described as new to science: Melanotopelia blepharostoma Lumbsch & Divakar, Topeliopsis athallina Lumbsch & Mangold and Topeliopsis patagonica Mangold & Lumbsch. The new combination Topeliopsis novae-zelandiae (Szatala) Lumbsch & Mangold is proposed.