An endolithic lichen is a
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 a ...
lichen that grows inside solid rock, growing between the grains, with only the fruiting bodies exposed to the air.
[Lichen Vocabulary, LICHENS OF NORTH AMERICA, Sylvia and Stephen Sharnoff]
An example is ''
Caloplaca luteominea
''Caloplaca luteominia'' subspecies ''bolanderi'' (ruby firedot) is an endolithic lichen, a crustose lichen that grows ''inside'' rock, between the grains, with only the ruby red fruiting bodies exposed to the air.Lichen Vocabulary, LICHENS OF ...
'' subspecies ''bolandri''.
References
Lichenology
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