Rhizocarpon
Rhizocarpon is a genus of crustose, saxicolous (or sometimes lichenicolous), lecideoid lichens in the family Rhizocarpaceae. The genus is common in arctic-alpine environments, but also occurs throughout temperate, subtropical, and even tropical regions. They are commonly known as map lichens because of the prothallus forming border-like bands between colonies in some species, like the common map lichen ("Rhizocarpon geographicum"). Taxonomy and phylogeny. Together with three small genera ("Catolechia", "Poeltinula" and "Epilichen"), "Rhizocarpon" constitutes the family Rhizocarpaceae. Historically, ca 389 names have been used. However, many species concepts are ill-defined, many names have been synonymized and new  species are regularly being described, so true number of species is not entirely clear as of now, but is estimated to be around 200. In molecular work, the genus has also been shown to be paraphyletic, with closely related genera being nested within "Rhizocarpon". Common traits. With so many species in a morphologically diverse genus it is difficult to say something general about morphology and anatomy and there will inevitably be some exceptions. However, they do share some key traits. They are all crustose and mostly saxicolous (rock-living), with some being lichenicolous (lichen parasites) on other saxicolous lichens. They  are all lecideoid, meaning they have apothecia without a thallus margin containing algae. Ascus and ascospores. The genus has a distinct type of ascus, the Rhizocarpon-type, which is bitunicate with the inner ascus-wall being slightly apically thickened. Ascospores are considered important characters for determining species within the genus. They are either 2-celled (1-septate) or multi-celled (muriform) and are either hyaline or pigmented (green or brown), often with a characteristic halo (or “perispore”) visible when viewed in a microscope. Asci contain eight, two or rarely one spore.