Wikipedia
Cryptothecia is a genus of white to greenish crustose lichens that grow on bark, wood, or leaves. in tropical or subtropical areas worldwide. It has a conspicuous prothallus that develops around its periphery which can be bright red in some species, hence the common name wreath lichen. The main vegetative body ( thallus) lacks a cortex ( ecorticate and is often immersed in the substrate or byssoid (whispy, like teased wool). The medulla is white, well defined, and often peppered with calcium oxalate crystals. Ascomata are not well defined, being cushions of soft white mycelium immersed in the medullary tissue, hence the name from the Greek “krypto” = “to conceal” and “theke” = “a container or sheath”. There are about 45 described species in the genus according to one source, and 75 species according to another. The genus is in the Arthoniaceae family. It contains Trentepohlia, a green alga, as its photobiont ).
Two species have been described in North America. At least one species, Cryptothecia rubrocincta, has been used in Brazil as a source of dye.