﻿Yol. 6 1.] SEQUENCE IN THE BRISTOL AREA. 183 



Throughout the Carboniferous Limestone of the Bristol area, 

 Corals and Brachiopods play the leading parts, and it is with these 

 two groups alone that this paper deals. 



The Bryozoans are, nevertheless, of considerable service in the 

 field, since they are abundant and undoubtedly obey a fixed law 

 of vertical succession ; but their zonal distribution must be worked 

 out by those who possess a special knowledge of this difficult 

 group. 



Since no system of zones, founded entirely upon a purely- 

 palaeontological basis, has previously been suggested for the Car- 

 boniferous Limestone, my task was rendered easier, in that I had 

 merely to adopt that system which best co-ordinated the faunal 

 succession in the Bristol area. Such a system, deduced from the 

 examination of a single area and founded entirely upon two fossil- 

 groups, cannot of course presume to be more than a preliminary 

 attempt to deal with a large and complicated problem ; but it may 

 serve as part of the scaffolding, by means of which a system of 

 general application will ultimately be built up. In order that the 

 suggested divisions may have more than a merely-local value, I 

 have selected genera for the zonal indices and species -groups 

 for the subzonal indices. 



The tendency of recent palseontological work is to make the 

 species the unit of division, and to include in its definition every 

 observable property. The necessary result of such minute definition 

 is, undoubtedly, to ensure that the same name always conveys 

 precisely the same meaning ; but, on the other hand, it tends to 

 confine each species to a limited area, as well as to a limited 

 horizon. Hence, a perfectly-defined species is not likely to be of 

 great value as a zonal or subzonal index. 



A gens or species-group may be considered to be the aggregate 

 of all the species which possess, in common, a large number of 

 essential properties, and are continuously related either in space 

 or time. I had originally used the term circulus in the sense here 

 defined; but, although Prof. Gregory certainly employs the term in 

 this sense, 1 I agree with Dr. Bather that this is not the original 

 meaning which he attached to the term. I consequently employ 

 circulus only in the sense explained by Dr. Bather, and suggest 

 the term gens to convey the meaning of a species-group as 

 here defined. (See Discussion, pp. 306-307.) 



It is immediately obvious that such a definition has no absolute 

 exactness, for it depends entirely upon the conception of what are 

 essential properties, and also upon how few such common 

 properties may be considered sufficient to allow of two species 

 being grouped in the same species-group. There is not, however, 

 any great difference of opinion as to the essential similarity of two 

 species ; the danger is, rather, that two species, evolved along 



1 ' Brit. Mus. Catal. of the Jurassic Bryozoa ' 1896, p. 27. 



o2 



