Polymeridium xanthoexcentricum

Polymeridium xanthoexcentricum is a species of corticolous (bark-dwelling), crustose lichen in the family Trypetheliaceae. It is found in southwestern Bolivia.

Taxonomy
The lichen was formally described as a new species in 2016 by lichenologists Adam Flakus and André Aptroot. The type specimen was collected in the Kaa-Iya del Gran Chaco National Park and Integrated Management Natural Area (Cordillera Province, Santa Cruz) at an elevation of 278 m; there it was found in the Chiquitano dry forests. It is only known to occur in this habitat in Bolivia.

Description
The thallus of the lichen is grey, smooth to somewhat, and covers an area of up to about 6 cm in diameter. It lacks both a and a. Although parts of the thallus are –immersed in the bark–the presence of the lichen does not induce the formation of galls in the host. The ascomata are more or less spherical, measuring 0.2–0.5 mm in diameter. Typically, they occur singly, although occasionally 2 or 3 will fuse together. The ostioles have an off-center placement. The lichen contains lichexanthone, which is a lichen product that causes the thallus to fluoresce a yellow colour when. The species epithet xanthoexcentricum refers to two main of this lichen: the presence of lichexanthone and the eccentric ostiolar placement.