Application of the term reef to archeocyathid accumulations is discussed. Archeocyathids occur as isolated individuals within the trilobite-brachiopod-hyolite faunal association of a fossiliferous zone, as an important element of more or less continuous massive beds, and as constituents of bioherms. In all cases the occurrences are of more primitive character than true reef formations. The archeocyathids do not form complexes characteristic of particular climatic and bathymetric conditions.