Abstract
Silurian graptolite zones are of markedly shorter duration than those of the Ordovician, averaging 0.8 Ma. Several can be sub-divided, and further refinement will undoubtedly follow a revision of many of the graptolite species and their stratigraphic range. Locally, integration of palaeontological and sedimentological studies facilitates correlation. A premilinary subdivision of the crispus and griestoniensis zones in central Wales is made.
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