Revised terminology is proposed that describes the ecological niches of microorganisms within hald, mineral substrates. Organisms attached to the external surfaces of the rock are termed epiliths, while those in the interior of the rock are all termed endoliths. The latter are called chasmoendoliths if they inhabit fissures in rocks, cryptoendotiths if they dwell within structural cavities, and euendoliths if they actively penetrate calcareous substrates.