Polymeridium is a genus of lichen-forming fungi in the family Trypetheliaceae. Species in the genus are corticolous (bark-dwelling) and typically occur in tropical areas.

Taxonomy
The genus was originally circumscribed as a section of the genus Arthopyrenia by Swiss lichenologist Johannes Müller Argoviensis in 1883. Richard Harris elevated it to generic status in 1980. In his 1993 revision of the genus, Harris identified 19 species. Utilizing a broad species concept, he acknowledged the presence of variations in chemistry and hamathecial  within individual species. André Aptroot and Marcela Cáceres again revised the genus in 2014, using the absence or presence of lichexanthone and the absence or presence of hamathecial inspersion as distinguishing characters for species. The monotypic genus Exiliseptum was incorporated into Polymeridium, as it only differs in one characteristic from several Polymeridium species and does not possess any unique features. As a result of this work, the number of species in the genus increased from 27 to 53.

Description
Polymeridium is characterized by a white to yellowish or grey thallus, which is usually ecorticate (i.e., it lacks a ). Its , the fruiting bodies of the lichen, are simple or fused, appearing as black, spherical to  structures that emerge from the . The ostiole, or opening of the ascomata, can be apical to lateral, brown to black, or exhibit a red anthraquinone colour.

The , which refers to the filamentous tissue within the ascomata, is colourless and may contain hyaline or red oil droplets that turn green when treated with potassium hydroxide (KOH) solution.  are the reproductive spores of the lichen and typically occur in groups of 4 to 8 per ascus. They are colourless, ellipsoid to  in shape, with rounded to subacute ends, and symmetrically 3 to 13- to . The ascospores are not constricted at the septa and are sometimes surrounded by a thin to 2–5 μm-thick gelatinous sheath. The septa within the ascospores are not thickened, and the  are rectangular and slightly rounded, but not diamond-shaped.

, or structures that produce  (asexual spores), are rather rare in the genus Polymeridium. In terms of secondary chemistry, the thallus occasionally contains lichexanthone, while anthraquinones are rarely present in the ostiole or hamathecium.

Species
Polymeridium albidoreagens 
Polymeridium albidovarians 
Polymeridium albidum 
Polymeridium albocinereum 
Polymeridium alboflavescens 
Polymeridium albopruinosum 
Polymeridium bambusicola  – Argentina
Polymeridium bengoanum 
Polymeridium brachysporum 
Polymeridium catapastoides 
Polymeridium catapastum 
Polymeridium contendens 
Polymeridium corticatum  – Brazil
Polymeridium costaricense 
Polymeridium endoflavens  – Brazil
Polymeridium fernandoi  – Sri Lanka
Polymeridium inspersum 
Polymeridium jordanii 
Polymeridium julelloides  – Brazil
Polymeridium longiflavens  – Brazil
Polymeridium microsporum 
Polymeridium multiforme 
Polymeridium multiseptatum 
Polymeridium neuwirthii 
Polymeridium ocellatum 
Polymeridium pyrenastroides 
Polymeridium pyrenuloides 
Polymeridium quinqueseptatum 
Polymeridium refertum 
Polymeridium rhodopruinosum  – Puerto Rico
Polymeridium siamense 
Polymeridium stramineoatrum 
Polymeridium submuriforme 
Polymeridium subvirescens 
Polymeridium suffusum 
Polymeridium sulphurescens 
Polymeridium tribulationis 
Polymeridium xanthoexcentricum  – Bolivia
Polymeridium xanthopleurothecium  – Panama
Polymeridium xanthoreagens

References

Trypetheliaceae
Dothideomycetes genera
Taxa described in 1883
Taxa named by Johannes Müller Argoviensis