﻿Vol. 6 I.] SEQUENCE IN THE BRISTOL AREA. 259 



in K x and again in S l . But it is a very suggestive fact that the 

 transition from the Upper Old "Red Sandstone into the Carboniferous 

 Limestone, whenever that transition is continuous, is in the Bristol 

 and surrounding areas always accompanied by this faunal phase. 



So far as I am capable of judging, the Modiola-like form which 

 occurs in S x is quite distinct from M. lata ; but, on the other hand, 

 I cannot distinguish the form which occurs in K x from the index- 

 form of the M odiola -Zone. Hence, it must be confessed that the 

 Modiola-Zone is not yet satisfactorily separated from the succeeding 

 zone by the presence of distinct forms, but is merely an earlier series 

 of beds in which the Cleistopora-£&un& makes its entrance. 



I do not, however, think that the Modio la-Zone should on this 

 account be merged in the succeeding zone ; for the Ehynchonellids 

 are, at least in the Avon section, distinct from those occurring 

 in the Gleistopora-Zone. It is also probable that some one among 

 the numerous small gasteropods, which are so characteristic of 

 these early beds, may ultimately afford a good zonal index. 



The Cleistopora-ZonQ. 



Whenever corals can be found, this zone is distinctly separated 

 off from the Zaphrentis-Zone above by the presence of Cleistopora 

 and the absence of Zaphrentis. To a certain extent, however, this 

 separation is a mere matter of definition. It rests upon the fact 

 that directly the early Zaphreutis-famna, is fully established, Za- 

 phrentis is found and Cleistopora is absent; but a sharp separation 

 of the two zones is impossible, for, throughout the upper part of 

 the Cleistopora-Zone, the brachiopods characteristic of the lower 

 Zaphrentis-Zone enter one by one, and become gradually and 

 continuously commoner. 



Hence, from a purely-paleeontological point of view, the separation 

 of the * Lower Limestone-Shales ' is a complete impossibility. The 

 faunal facies of the included limestones, at the top of the ' shales,' 

 is practically identical with that of the lowest beds of the massive 

 Limestone-Series, and the faunal continuity from the one into the 

 other is perfect. Furthermore, the small variations a/t different 

 points of the area in the faunal contents of the lowest beds of the 

 massive Limestone-Series indicates that the junction of shales and 

 limestone is not exactly at the same horizon throughout the area. 



The bassus- and octoplicata-suhzones. 



These two subzones are the expression of an important change of 

 facie*. 



The b as s us -sxjl\> zone contains: — 



(1) Representatives of the earlier Modiola-famm: for example, 



the same type of Retzid and the same form of Cliothyris 

 Boyssii. 



(2) Characteristic forms, such as Productus bassus, Chonetes 



' Buchiana. 9 



(3) A characteristic fish-fauna (see above, p. 253). 



