﻿258 DR. A. VAT7GHAN 0^ THE PALJS0NT0I0GICAL [May I905, 



first approximation, it has been demonstrated, beyond all question, 

 that the relative position of the various groups is constant 

 throughout the whole of the Bristol area : that is, that the species 

 succeed each other always in the same order. 



This statement holds true to a much higher degree of approxi- 

 mation, when we consider the corals alone, or the brachiopods 

 alone. In fact, the only deviations from absolute constancy in this 

 case consist in displacements in the positions of the maxima, as we 

 pass from one point of the area to another. When, however, 

 we measure the relative position of the corals by the brachiopod- 

 sequence, or vice versa, there is a more important deviation which, 

 although unimportant in a small area, would, if continued in the 

 same sense to a considerable distance, need to be carefully allowed for. 



We can therefore obtain a second approximation which shall 

 hold true at considerable distances, by leaving the relative suc- 

 cession of the brachiopods unaltered, as also the relative succession 

 of the corals, but by slightly displacing the one class relatively to 

 the other. Very much more work must, however, be done in the 

 Bristol and neighbouring areas (for example, in the Mendips and 

 in South Wales) before the exact law of relative acceleration can 

 be fixed. 



In so far as the Bristol area is concerned, the table of ranges is 

 sufficient to enable any worker to determine the horizon of any 

 exposure which lies within the area, to a considerable degree of 

 accuracy. If he registers all the fossils that he sees, and also notes 

 their absolute and relative abundance, he will have no difficulty 

 in determining, from the diagrams, the approximate horizon at 

 which the exposure lies. I have tested this over and over again, 

 and the results have always agreed with the position of the exposure 

 as determined by stratigraphical reasoning (whenever such reasoning- 

 was possible). Before studying the neighbouring areas, I claimed 

 that the main object of my prolonged work was to this extent 

 achieved, that any fossiliferous exposure, lying within the Bristol 

 area, could be horizoned to an adequate degree of approximation 

 by the use of the range-tables ; and I now go farther, and claim that 

 the sequence established for the Bristol area holds good 

 throughout the South-West of England and in South 

 Wales. 



Analysis of the Faunal Characters of the Zones 

 and Subzones, 

 The Modiola-Z<me. 



This zone is a ipre-Cleistojtora-Zone, in which the typical Cleisto- 

 jwm-fauna makes its entry, but in which this fauna is as yet very 

 incompletely represented. The occurrence of Modiola-like forms 

 associated with ostracods and Sjnrorbis does not in itself indicate 

 any particular horizon, but rather marks a phase of sedimentation. 

 The same association is to be found at several horizons in the 

 Carboniferous Limestone of the neighbouring areas : for example, 



