Astrothelium lucidothallinum
Astrothelium lucidothallinum is a species of corticolous, crustose lichen in the family Trypetheliaceae. Found in Guyana, it was formally described as a new species in 2016 by Dutch lichenologist André Aptroot. The type specimen was collected about south of Aishalton at an altitude of ; there, it was found in a savanna growing on smooth tree bark. The lichen has a smooth and somewhat shiny, pale yellowish grey thallus with a cortex and a thin black prothallus line. It covers areas of up to in diameter. The thallus contains lichexanthone, a lichen product that causes it to fluoresce yellow when lit with a long-wavelength UV light. The combination of characteristics of the lichen that distinguish it from others in Astrothelium are: the presence of lichexanthone only in the thallus; the indistinctly pseudostromatic ascomata, with pseudostromata—whitish in colour but lacking a sharp outline; and the dimensions of the ascospores as well as their number per ascus.