﻿Vol. 6 1.] SEQUENCE IN THE BRISTOL AREA. 307 



questions were treated in this way, although he had lately heard 

 the opinion expressed that the Carboniferous Limestone did not 

 seem to lend itself to zoning. 



The Author thanked the Fellows for their nattering reception of 

 his paper, and expressed his especial gratitude to those who had 

 taken part in the discussion, for their very kind remarks. In answer 

 to Mr. H. B. Woodward, he regretted that he had not been able to 

 find roDm, in a short abstract, either for a correlation of the zones, 

 suggested in his paper, with the earlier lithological divisions, or for 

 a reference to the work done by the late W. W. Stoddart in the 

 Avon section. Both these subjects were, however, fully dealt with 

 in the paper itself. In reply to Dr. Bather, he said that the term 

 circulus was employed, throughout his paper, in a very definite 

 sense. A circulus included all the forms which were very closely 

 related to the type-species and were in perfect continuity with 

 that species, either horizontally or vertically; it excluded all 

 apparently-similar forms which were separated by a long vertical 

 interval from the occurrence of the type, and were not connected 

 with the type by a chain of similar forms. The Author had adopted 

 the term circulus from Prof. Gregory's work on the Jurassic 

 Bryozoa under the same impression as the Rev. J. F. Blake, that it 

 very conveniently covered the whole group, which was composed of 

 the type-species and its close relatives. Should, however, Dr. Bather's 

 interpretation be the correct one, namely, that the term circulus 

 was originally intended to cover a heterogeneous assemblage of 

 similar forms, for which even genetic relationship could not be 

 claimed, a new term would have to be introduced to" convey the 

 meaning in which the term circulus was employed in the present 

 paper. 



With regard to the use of the term mutation, it was undoubtedly 

 employed in this paper to denote lateral, as well as vertical, varia- 

 tion from a type-species. The Author considered that both lateral 

 and vertical variation were equally cases of evolutionary change, 

 seeing that at least one important factor in lateral variation was 

 direct evolution during migration. It would be impossible to dis- 

 criminate between the action of this factor and of the other 

 recognized cause, namely, direct evolution from distinct members of 

 a circulus, which took place contemporaneously at distant points. 



