﻿Vol. 6l.] SEQUENCE IN THE BKISTOL AREA. 277 



separate the different forms which I have tentatively referred to 

 this group. All the specimens that I have collected occur in the 

 Dibunophyllum-Zone. 



Form (1) has a narrow external area, of a Lithostrotion-type 

 of structure. Form (2) is characterized by strong rugae and a thick 

 outer wall ; the septal ring is separated from the outer wall by 

 very large vesicles. The septa are well spaced. 



Lithostrotion. 



Until a very much larger series of accurately-horizoned specimens 

 has been collected and studied, it will be vain to attempt a natural 

 classification of this group founded upon true evolutionary modi- 

 fication. I have, consequently, adhered, as closely as possible, 

 to the artificial system of species recognized by Edwards & Haime 

 in their ' Monograph of the British Fossil Corals.' It is, however, 

 important to draw attention to the principal modifications which 

 affect the typical Lithostrotion-structure. 



The typical Lithostrotion-stru.ctu.re may be defined as follows : — 



(1) Well-spaced, alternate septa, all of which extend to the wall; all the 



septa are of nearly-equal thickness, and each septum varies very little in 

 thickness along its length. 



(2) An external vesicular area in a horizontal section. This area forms a 



ring, of uniform width, within the wall ; it is radiated by both series of 

 septa, and the interseptal spaces are each occupied by one or more 

 vesicles ; the inner boundary forms the inner wall of the corallite. 



(3) The tabula? are conical or gently dome-shaped, without any definite 



fossular depression ; the septa extend inwards, for a greater or less 

 distance, along the upper faces of the tabulae. 



(4) From the centre of each tabula, an erect, laterally-compressed columella 



projects upward, and usually reaches the tabula immediately above. 



The following are the most important structural modifications : — 



(1) Diphyphyllidan modification. 



The columella is either absent, or does not stretch from tabula to 

 tabula. This type of structure is always associated with very 

 broad, flattened tabulae. It is common in the group of Lithostrotion 

 Martini, but is extremely rare in L. irregulare and throughout the 

 Nematophyllwm-$ec,tion. 



In this modification, the septa stop well short of the centre. 



(2) Koninckophyllidan modification (compare the structure 



of Koninckophyllum 0, PI. XXIII, fig. 4). 

 The external vesicular area is very broad, and the interseptal 

 spaces are crowded with small vesicles. Lithostrotion affine and 

 the whole Nernatophyllum-&QQ,tion are examples of this type of 

 structure. 



(3) Clisiophyllidan modification. (PI. XXIII, fig. 5.) 



In a horizontal section, the central area shows the characteristic 

 spider-web structure. This type of structure results from the 

 shape of the tabulae, which are strongly conical and radiated by a 

 regular series of lamellae. 



Q. J. G. S. No. 242. u 



