Arthothelium

Arthothelium is a genus of lichen-forming fungi in the family Arthoniaceae. Species in the genus typically form crusts on smooth bark in humid, undisturbed habitats. They are distinguished from the related genus Arthonia by their spores, which are divided by both transverse and longitudinal walls into a brick-like pattern. The genus has a cosmopolitan distribution, with most species occurring in tropical regions.

==Taxonomy== Arthothelium was introduced as a new genus by the Italian lichenologist Abramo Bartolommeo Massalongo in 1852, for species that earlier authors had treated among genera such as Arthonia, Graphis, and Opegrapha. In the protologue, Massalongo defined the genus by its gelatinous apothecia, which are concealed by the thallus when young but later become swollen, misshapen, and rough-wrinkled, and by ovoid to pear-shaped asci bearing 6–8 spores with only barely visible paraphyses. He described the spores as ovoid-ellipsoid and initially colourless, then increasingly divided by 8–10 transverse septa and 1–2 longitudinal septa (forming many internal "cells", or ), and finally darkening to a sooty brown at maturity; the thallus itself was described as a uniform, closely attached crust.