Discussion

Abstract

Dr W. S. McKerrow and Dr L. R. M. Cocks congratulated the author on his assessment of the palaeogeography of this area. They observed, however, that the maps showed a contour at a depth of 100 metres, and wondered what criteria had been used to estimate this depth? The distribution of the brachiopod communities of Ziegler, Cocks & Bambach appears to be closely related to depth, but so far there has been no satisfactory method of determining the actual depth quantitatively. The evidence of changes in community above and below lava flows in Pembroke and Gloucestershire now appears to be ambiguous. If the author's submarine contours reflect the boundary between the Stricklandia and Clorinda Communities, or perhaps below the Clorinda Community, this should be stated with reference to these communities, rather than to any particular depth in metres. Dr Cocks also drew attention to the importance of the crenulata Zone graptolite fauna from the base of the Upper Owenduff Group near Doon Rock. This is stratigraphically above horizons with Eocoelia sulcata, and is yet another link between the sequence of graptolite zones and the evolving brachiopod lineages which have proved so useful for correlation within the Lower Silurian. Associated with the graptolites were rare brachiopods, and one of these, Pentlandella pentlandica (Haswell 1865), is notable since it was known previously only from its type locality in the Pentland Hills, Scotland, which may also be of crenulata Zone age. In a written reply, the A uthor thanked Dr McKerrow and Dr.Cocks