﻿Yol. 6 I.] SEQUENCE W THE BRISTOL AREA. 189 



(ii) The presence of forms which, in the Bristol area, are confined to the 

 upper portion of Zone M and to Subzone K. Examples are: — 

 Eumetria (' Retzia ') spp. ; Cliothyris Royssii ; Camarotcechia aff. 

 mitcheldeanensis. 



If account be taken of the other groups : — 



The rare occurrence of Modiola lata and of ostracods in Horizon a] links 

 that horizon with Zone M. 



The abundance of Ehabdomeson links Horizon a with Zone K. Hence 

 Horizon a forms, from a palasontological point of view, a somewhat indefinite 

 level of overlap, in which the Modiola-ostv&cod phase passes gradually into the 

 faunal phase characteristic of the Cleistopora-Zone. 



From a lithological standpoint, the development of the highly- 

 characteristic red, crystalline, encriuital limestone at this level 

 throughout the Bristol area appears to give to Horizon a a definite- 

 ness which its palseontological characters scarcely warrant. 



K = Zone of Cleistopora aff. geometrica 

 ( CzmsTOPOBA-Zone) . 



Lithological character. — This zone has the 'Palate-Bed' for 

 its base, and includes the whole of the thick series of shales (with 

 subsidiary limestones) which lie between that bed and the base 

 of the massive limestones of the Zaphrentis-Zone. 



Former designation. — The whole of the Lower Limestone- 

 Shales which lie above the ' Bryozoa-Beds.' 



Where exposed in the Avon section: — 



Clifton side: Occasional exposures along the Avonmouth line, 



near Cook's Folly. 

 Leigh Woods side : The lowest beds are shown in the riverside 



exposure ; the topmost beds are shown in Quarry 1 (see 



PI. XXVII). 



Coral-fauna : 



Cleistopora aff. geometrica, 

 Brachiopod-fauna: 



Cliothyris Royssii vars. & mut. /3. 

 Spirifer aff. clathratus & var. 

 Spiriferina octoplicata. 

 Syringothyris aff. cuspidata. 

 Eumetria (' Retzia ') aff. carbonaria. 

 Camaroto3chia mitcheldeanensis. 

 Leptena analoga. 



Orthotetes crenistria (especially 

 mut. K x ). 



Subdivisions : — 



K x = Subzone of Pro ductus bassus (BAssus-subzone) . 



This includes only the lowest portion of the zone, and is charac- 

 terized by the great abundance of : — 



Camarotcechia mitcheldeanensis, Leptena, Orthotetes crenistria, mut. K,, 

 Productus bassus, and small Chonctes. 



Rhipidomella aff. Michelini. 



Productus bassus. 



Productus cf. Martini. 



Chonetes cf. Buchiana, Ch. cf. crassi- 



stria, and intermediate forms. 

 Chonetes cf. hardrensis. 

 Lingula sp. 

 (JDiscina) sp. 



