''Placidium'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

crustose Crustose is a habit of some types of algae and lichens in which the organism grows tightly appressed to a substrate, forming a biological layer. ''Crustose'' adheres very closely to the substrates at all points. ''Crustose'' is found on rocks and ...

squamulose A squamulose lichen is a lichen that is composed of small, often overlapping "scales" called squamules. If they are raised from the substrate and appear leafy, the lichen may appear to be a foliose lichen, but the underside does not have a "skin ...

to almost

foliose Foliose lichen is one of the morphological classes of lichens, which are complex organisms that arise from the symbiotic relationship between fungi and a photosynthetic partner, typically algae. This partnership allows lichen to live in diverse ...

lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.Verrucariaceae The Verrucariaceae are a family of mostly lichenised fungi in the order Verrucariales. The lichen-forming species, which comprise the vast majority of the family, have a wide variety of thallus forms, and include crustose (crust-like), foliose ( ...

. Most members grow on soil (are

terricolous A terricolous lichen is a lichen that grows on the soil as a substrate. An example is some members of the genus ''Peltigera ''Peltigera'' is a genus of approximately 100 species of foliose lichens in the family Peltigeraceae. Commonly known as ...

), but some grow on rock (

saxicolous A saxicolous lichen is a lichen that grows on rock. The prefix "sax" from the Latin means "rock" or "stone". Characteristics Saxicolous lichens exhibit very slow growth rates. They may develop on rock substrates for long periods of time, give ...

). The fruiting bodies are

perithecia An ascocarp, or ascoma (), is the fruiting body ( sporocarp) of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores. Ascocarps are mo ...

, flask-like structures immersed in the lichen body (

thallus Thallus (plural: thalli), from Latinized Greek (), meaning "a green shoot" or "twig", is the vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria. Many of these organisms wer ...

) with only the top opening visible, dotting the thallus.

Lichen spot test A spot test in lichenology is a spot analysis used to help identify lichens. It is performed by placing a drop of a chemical on different parts of the lichen and noting the colour change (or lack thereof) associated with application of the chemical ...

s are all negative. Members of the genus lack

rhizines In lichens, rhizines are multicellular root-like structures, arising mostly from the lower surface. A lichen with rhizines is termed rhizinate, while a lichen lacking rhizines is termed erhizinate. Rhizines serve only to anchor the lichen to their ...

, but otherwise resemble members of the genus ''

Clavascidium ''Clavascidium'' is a genus of lichens in the family Verrucariaceae. The genus was circumscribed in 1996 by Austrian lichenologist Othmar Breuss. Because the type species of the genus, ''Clavascidium umbrinum'', has been shown using molecular ...

''. Members of the genus are commonly called stiplescale lichens or earthscale.Field Guide to California Lichens, Stephen Sharnoff, Yale University Press, 2014,

Taxonomy

The genus was

circumscribed In geometry, the circumscribed circle or circumcircle of a polygon is a circle that passes through all the vertices of the polygon. The center of this circle is called the circumcenter and its radius is called the circumradius. Not every polyg ...

in 1855 by Italian lichenologist

Abramo Bartolommeo Massalongo Abramo Bartolommeo Massalongo (13 May 1824 – 25 May 1860) was an Italian paleobotanist and lichenologist. He was born in Tregnago in the Province of Verona and took a great interest in botany as a young man. Massalongo joined the faculty of med ...

. María Prieto and Ibai Olariaga suggest that ''Placidium'' is a superfluous illegitimate name according to strict application of nomenclatural rules. This is because when Massalongo circumscribed the genus in 1855, he included the original

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...

of genus ''

Endocarpon ''Endocarpon'' is a genus of saxicolous (rock-dwelling), crustose lichens in the family Verrucariaceae. The genus was circumscribed by German bryologist Johann Hedwig Johann Hedwig (8 December 1730 – 18 February 1799), also styled as Johanne ...

'', '' E. pusillum''. When Othmar Breuss resurrected the genus for use in 1996, he assigned ''P. michelii'' as its type, even though Massalongo did not include ''P. michelii'' in ''Placidium'' in his original 1855 circumscription, but rather in a later treatment of the genus. In 2019, Prieto and Olariaga submitted a proposal to

conserve Conserve may refer to: * Conserve (condiment), a preserve made from a mixture of fruits or vegetables * Conserve (NGO), an Indian environmental organization * Conserve (publisher), a Dutch publisher * Conserved sequence, a protein or nucleic aci ...

the name ''Placidium'' with ''Placidium michelii'' as its conserved

Species

* '' Placidium arboreum'' * '' Placidium californicum'' – USA * '' Placidium lesdainii'' * ''

Placidium nigrum ''Placidium nigrum'' is a species of squamulose (scaley), ground-dwelling lichen in the family Verrucariaceae. It is found in Northwest China and the Tibetan Plateau, where it grows on sandy soil surfaces in semi-arid and arid regions. The lich ...

'' – China * ''

Placidium nitidulum ''Placidium'' is a genus of crustose to squamulose to almost foliose lichens. The genus is in the family Verrucariaceae. Most members grow on soil (are terricolous), but some grow on rock (saxicolous). The fruiting bodies are perithecia, flask-l ...

'' – China * ''

Placidium podolepis ''Placidium'' is a genus of crustose to squamulose to almost foliose lichens. The genus is in the family Verrucariaceae. Most members grow on soil (are terricolous), but some grow on rock (saxicolous). The fruiting bodies are perithecia, flask-l ...

'' * ''

Placidium umbrinum ''Placidium'' is a genus of crustose to squamulose to almost foliose lichens. The genus is in the family Verrucariaceae. Most members grow on soil (are terricolous), but some grow on rock (saxicolous). The fruiting bodies are perithecia, flask-l ...

'' * ''

Placidium varium ''Placidium'' is a genus of crustose to squamulose to almost foliose lichens. The genus is in the family Verrucariaceae. Most members grow on soil (are terricolous), but some grow on rock (saxicolous). The fruiting bodies are perithecia, flask-l ...

'' – China * ''

Placidium yoshimurae ''Placidium'' is a genus of crustose lichen, crustose to squamulose lichen, squamulose to almost foliose lichen, foliose lichens. The genus is in the family Verrucariaceae. Most members grow on soil (are terricolous), but some grow on rock (saxi ...

References

{{Taxonbar, from=Q7200385 Verrucariales Eurotiomycetes genera Lichen genera Taxa described in 1855 Taxa named by Abramo Bartolommeo Massalongo