conjunction with the British Geological Survey’s Central

Siurian graptolite zones are of markedly shorter duration than those of the Ordovician, averaging 0.8Ma. Several am be sub-divided, and further refinement will undoubtedly follow a revision of many of the graptolite species and their stratigraphic range. Lacally, integration of palaeontological and sedimentological studies facilitates correlation. A preliminary subdivision of the crispus and griestoniensis zones in central Wales is made. Graptolite biostratigraphy has played an important part in the development of the understanding of the Silurian geology of the Lower Palaeozoic Basin of Wales. Over half of the Silurian graptolite zones in current use were originall