﻿Vol. 6 1.] SEQUENCE IN THE BEISTOL AREA. 191 



Subdivisions : — 

 j3= Horizon /3. 



This includes the very lowest beds of this zone ; these beds 

 might, probably with equal truth, be considered as the very 

 topmost beds of the preceding zone, for Zaphrentis and Spiriferina 

 octoplicata co-occur here, while all the brachiopods pass con- 

 tinuously from the series below into this horizon. It is, then, a 

 well-marked horizon of overlap, and is lettered accordingly. 



I include it as the base of the Zaphrentis-Zone rather than as 

 the top of the Cleistopor a -Zone, for three reasons : — 



(a) Zaphrentis is first met with here, and is immediately fairly common, 



whereas Cleistopora has not been registered here. 



(b) Cliothyris glabristria first occurs (so far as I know) at this horizon. 1 



(c) Spirifer aff. clathratus becomes extraordinarily abundant, and con- 



tinues so for some time. 



Where exposed in the Avon section: — 



Clifton side : The lowest part of the Black Rock (that is, the small 

 separate quarry, which is now closed and forms part of a 

 private estate). 2 

 Leigh Woods side : Quarry 1 . 



Special faunal characters. — This horizon is only to be dis- 

 tinguished from the rest of the lower portion of the Zaphrentis- 

 Zone by the common occurrence of Spiriferina octoplicata ; 

 all its other characters are exactly those which distinguish the 

 clathratus- subzone. 



Z x = Subzone of Spirifer aff. clathratus (clathbatus- 

 subzone) . 



[So named from the enormous abundance of the index-fossil.] 



Where exposed in the Avon section: — 



Clifton side : All the beds of the Black Rock, up to the slope at 



the northern end of the present quarry. 

 Leigh Woods side : Quarry 1, and the lowest part of Quarry 2. 



Special faunal characters : — 

 Brachiopods : 



Cliothyris Boyssii (mut. /3), RhipidomeUa, and Productus cf. 

 Martini reach their maxima, while Spirifer aff. clathratus and 

 its varieties maintain their extreme abundance throughout the 

 subzone. 



Spiriferina octoplicata, as already stated, is rare above the 

 lowest beds (Horizon /3). 



Rhynchonella mitcheldeanensis, Leptena analoga, Chonetes cf. 

 hardrensis, and Oh. cf. crassistria are abundant at certain levels. 



1 See note under Cliothyris glabristria, p. 297. 



2 This quarry is named 'Press's Quarry,' on the sketch-map of the Avon 

 quarries (PI. XXVII). 



