''Circinaria arida'' (pebble ball lichen) is a 0.5–6 cm, light olive-brown

crustose lichen Crustose lichens are lichens that form a crust which strongly adheres to the Substrate (biology), substrate (soil, rock, tree bark, etc.), making separation from the substrate impossible without destruction. The basic structure of crustose lichen ...

that grows on rock, often like a cluster of little light brown to olive balls growing on pebbles, in the southwestern deserts of North America. It is also found in

Eurasia Eurasia ( , ) is a continental area on Earth, comprising all of Europe and Asia. According to some geographers, Physical geography, physiographically, Eurasia is a single supercontinent. The concept of Europe and Asia as distinct continents d ...

, and arid parts of North America from the southern

Great Plains The Great Plains is a broad expanse of plain, flatland in North America. The region stretches east of the Rocky Mountains, much of it covered in prairie, steppe, and grassland. They are the western part of the Interior Plains, which include th ...

and

Midwest The Midwestern United States (also referred to as the Midwest, the Heartland or the American Midwest) is one of the four census regions defined by the United States Census Bureau. It occupies the northern central part of the United States. It ...

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

.Field Guide to California Lichens, Stephen Sharnoff, Yale University Press, 2014, , p. 255Lichen Flora of the Greater Sonoran Desert Region. Vol 3, Nash, T.H., Ryan, B.D., Gries, C., Bugartz, F., (eds.) 2001

/ref> It is warty (

verrucose This glossary provides an overview of scientific terminology, terms used in the description of lichens, composite organisms arising from algae or cyanobacteria living symbiosis, symbiotically among Hypha, filaments of multiple fungus species. Er ...

) with the warts sometimes cracking apart

areolate Lichens are symbiotic organisms made up of multiple species: a fungus, one or more photobionts (an alga and/or a cyanobacteria) and sometimes a yeast. They are regularly grouped by their external appearance – a characteristic known as their gro ...

. The warts or areolas have angular to rounded sides. The 0.2-2.3 mm, convex to flat-topped areolas are separated by deep fissures that may be as deep (0.1–2 mm) as the areola is wide, so the lichen often appears to be made of clusters of little balls crammed up next to each other, although the areolas are sometimes isolated. In

it is commonly found growing on pebbles. Each areola has a single sunken black, dust covered (

pruinose Pruinescence , or pruinosity, is a "frosted" or dusty-looking coating on top of a surface. It may also be called a pruina (plural: ''pruinae''), from the Latin word for hoarfrost. The adjectival form is pruinose . Entomology In insects, a "bloom" ...

) fruiting body (

apothecium An ascocarp, or ascoma (: ascomata), 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. Ascoc ...

) with a white rim. A thin strip of

prothallus A prothallus, or prothallium, (from Latin ''pro'' = forwards and Greek ''θαλλος'' (''thallos'') = twig) is usually the gametophyte stage in the life of a fern or other pteridophyte. Occasionally the term is also used to describe the young ...

sometimes is at the outer edge, forming a narrow dark zone (fimbriate). The similar '' Aspicilia desertorum'' has a white pruina (dusty coating) on the apothecia. It is negative for

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 reagent on different parts of the lichen and noting the colour change (or lack thereof) associated with application of the ...

s, I−, K−, P−, C−.

References

Megasporaceae Lichen species Lichens described in 2011 Lichens of Asia Lichens of Europe Lichens of North America Taxa named by Leif Tibell {{Pertusariales-stub