﻿Vol. 6 1.] SEQUENCE IN THE BRISTOL AREA. 261 



(2) If we take the limits of the Caninia-Zone to be defined by the two 

 maxima of the gens C. cylindrica in the Avon section, the lower part of the 

 zone contains a fauna which is characteristically Tournaisian, while the upper 

 part exhibits a typically Visean facies. 1 



The Caninia-Zone has consequently been divided into a lower 

 subzone, the Syringothyris-Zone, which forms the top of the Tour- 

 naisian ; and au upper subzone, which forms the base of the Visean 

 and is included in the Seminula-Zone, of which it forms the base. 



The Syringothyris-Zone. 



This zone is characterized by the dying-out of the Tournaisian 

 fauna and the evolution of the Visean. 



Examples : 



(1) Ortkotetes crenistria occurs in profusion, as it does throughout the 



Tournaisian, but is associated with abundant Chonetes papilionacea. 



(2) Caninia cylindrica, which is enormously abundant at the base, gives 



rise by mutational change to the earliest member of the gens of 

 Cyathophylluiii MurchisQiii (namely, Cyathopkyllum <p). 



(3) Productus Cora becomes more abundant, and Semimda appears for the 



first time. 



(4) The .earliest forms of Lithostrotion and of the Clisiophyllids occur 



very sparingly. 



The zone is further characterized by the dominance of Syringo- 

 tliyris, which is represented by typical forms of S. cusjoidata and 

 S. aff. laminosa. 



The Seminula-Zone. 



The abundance in individuals of Lithostrotion, Seminula, and 

 giganteid Productus may be said to typify the Visean facies. 



The Seminula-Zone is characterized by recurring layers, made up 

 almost entirely of the shells of Semimda. This zone is naturally 

 divided into two subzones : — 



The lower subzone or semireticulatus-suhzone, in 

 which the Seminula-i&mm contains a few survivors of the Tour- 

 naisian facies, such as Caninia, Syringothyris, Productus semi- 

 reticidatus. 



The upper subzone or Cora-subzone, in which the 

 Dibunophyttum-f&una, is making its appearance. This subzone 

 is characterized by the occurrence of a particular mutation of the 

 gens of Productus Cora. 



The Dibunophyllum-Zone. 



This zone is one of the easiest to distinguish, on account of its 

 characteristic coral-fauna. 



The attempt to separate an ' Upper Limestone-Shale ' division, 

 starting at the point where shales become prevalent in the Avon 



1 To avoid any interruption of the zonal discussion, the distinctness of these 

 two faunas is demonstrated in a separate note. 



Q. J. G. S. No. 242. t 



