﻿Vol. 6 1.] SEQUENCE IN THE BRISTOL AREA. 281 



These genera may be grouped in sections, as follows : — 



Section I. (The Koninckophyllum-Group.) 



This group is composed of Clisiophyllids in which the central 

 area contains a thickened mesial plate, surrounded by a very 

 simple network of few meshes. 



Here Koninclcophyllum, Aceophyllum. and Acrophyllum may, for 

 the present, be placed together. 



Koninckophylltjm, Nich. & Thorns., may be regarded as having 

 close affinity to Lithostrotion, combined with a Clisiophyllidan 

 tendency. 

 The affinity to Lithostrotion is shown in : — 



(1) The lath-like columella, and the simplicity of the structure of the 



central area. 



(2) The marked ly-alternate septa ; the two series are of almost-equal 

 strength, and each septum is nearly uniform in thickness throughout 

 its length. 



(3) The secondary septa reach almost, or quite, to the inner wall. 



The Clisiophyllidan tendency is conspicuous in : — 



(1) The very broad external area, closely packed with vesicles. 



(2) An extremely-simple type of Clisiophyllid-structure^. in the central 



area. 



Axophtllum (Edwards & Haime), Thomson. 



This genus has apparent resemblance both to Koninckophyllum 

 and to Lonsdalia^ but its true affinity is to the latter. 

 It resembles Lonsdalia in : — 



(1) The highly-specialized Clisiophyllid-structure of the external area. 



(See under JDibunophyllum \l>, p. 284.) 



(2) The development of a purely-vesicular peripheral area. 



It apparently resembles Koninckophyllum in having a thick 

 columella, surrounded by a very simple network ; but, in the only 

 form from the Bristol area that I include under this genus, the 

 columella, when examined under the microscope, is seen to^be 

 complex in structure and to contain, in itself, the probable repre- 

 sentative of the entire central area of a Lonsdalia. Hence, the true 

 place of this genus is probably in Section V (p. 286.) 



Acrophyllum, Nich. & Thorns. 



This genus I propose to omit, at least for my present purpose. 



Those species which only differ from Koninclcophyllum in the 

 greater extension of the mesial plate across the central area, may 

 well be placed in the genus Koninclcophyllum ; while those species, 

 in which the external and peripheral areas exhibit a highly- 

 specialized Clisiophyllid-structure, may be included in the genus 

 Axophy Hum. 



This separation will also be in accord with the periods at which 

 the two divisions of Acrophyllum attained their maximum de- 

 velopment ; for Koninclcophyllum has its maximum in D v whereas 

 Axophyllum is confined to D 2 . 



