The problems of defining biozones are discussed, and the definitions of Holland et al. 1978 are considered unsatisfactory. An ideal methodology of biostratigraphical correlation is outlined and is contrasted with existing practice as exemplified by recent decisions of the Subcommission on Silurian Stratigraphy relating to stages of the Llandovery series. The numbers of species in the different Llandovery graptolite biozones and the proportions of zonally diagnostic species at the inter-biozone boundaries are tabulated. The Llandovery area is considered unsuitable for the establishment of stages, and the stratotypes for the Aeronian and Telychian are considered unsatisfactorily defined in relation to graptolite zonation.

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