. Bulletin of the British Museum (Natural History), Geology. 6 COCKS & RICKARDS correlations about the boundary, and that is its seemingly poor record of graptolites. It may be that at some future time graptolites may be relatively demoted in value for correlative purposes, but that time is still far away on present information. Dob's Linn also has most of the attributes of a boundary stratotype, and the Working Group, after eleven years of study, considered it better than Anticosti. In fact, the boundary has now been certainly put at the correct level, using the best group for correlation
. Bulletin of the British Museum (Natural History), Geology. 6 COCKS & RICKARDS correlations about the boundary, and that is its seemingly poor record of graptolites. It may be that at some future time graptolites may be relatively demoted in value for correlative purposes, but that time is still far away on present information. Dob's Linn also has most of the attributes of a boundary stratotype, and the Working Group, after eleven years of study, considered it better than Anticosti. In fact, the boundary has now been certainly put at the correct level, using the best group for correlation, the graptolites. Despite the fact that the Hirnantia brachio- pod fauna is very often overlain by persculptus Zone graptolites, unequivocal evidence from both Kazakhstan (Koren et al. this volume) and the Lake District of England (Cocks this volume) shows that it also occurs rarely within the persculptus Zone. There is a strong feeling amongst most biostratigraphers that they prefer to regard the Hirnantia fauna as Ordovician rather than Silurian in age and not straddling the systematic boundary, and this assignment to the Ordovician can be achieved only by a sub-acuminatus Zone boundary, as was eventually decided. A more interesting question is the precise age, in terms of graptolite zones, of the maximum glacio-eustatic drop in sea level, and this is still not yet definitively answered although it was probably about half way through the persculptus Zone—there are some well-dated persculptus bearing post-glacial transgressive beds in parts of North Africa. On the other hand, the precise duration and extent of the glacial episode (Fig. 1) certainly varied from place to place— commencing even in late Caradoc and early Ashgill times in some parts of Gondwana, and certainly continuing into post-Hirnantia fauna times, perhaps into the Rhuddanian, in others, South Africa. It is also important to note that detailed investigation indicates that the 'end. Fig. 1 Distribution
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