﻿Vol. 6 1.] SEQUENCE IN THE BRISTOL AEEA. 283 



Glasgow, vol. xiv (1883), and the sections of Dibunophyllum, shown in pi. xii, 

 fig. 1, ibid. 



(2) The helmet-shaped boss is a phenomenon common to all crested Clisio- 

 phyllids ; it depends upon the fact that a more prominent lamella runs down 

 the tent-shaped tabulse, from one end of the crest into the fossula, whereas the 

 other end of the crest usually is sharply terminated and not continuous with a 

 strong lamella (compare Koninckophyllum 9, p. 282). 



The distinction between Dibunophyllum and Qlisiophyllum seems 

 to be of more practical utility. 



Olisiophyllum includes all the Clisiophyllids of this section, in 

 which the mesial plate is wholly included within the central 

 area ; and in which the radial lamellae are all thin and of similar 

 character, so that the area presents a radial, rather than an axial 

 symmetry. 



Dibunophyllum includes all the Clisiophyllids of this section, in 

 which the thickened mesial plate extends nearly across the central 

 area, and is continued, as a more prominent lamella, into the 

 fossula. 



The distinction is, however, obviously only one of degree. 



Examples from the Bristol Area. 

 Dibunophyllum. 



Dibunophyllum Q. (PI. XXIV, fig. 1.) 



The form is cylindrical. 



Horizontal section. — The central area is composed of open 

 vesicles, and is bisected by a distinct, continuous plate, which is 

 nowhere greatly thickened. 



The primary septa, 40 to 50 in number, are well-spaced, not 

 specially thickened, and distinctly traceable, almost to the outer 

 wall. The secondary septa are either absent or inconspicuous, 

 extending only a short distance inwards from the outer wall. 



The septal break (' fossula ') is well marked, and the prolongation 

 of the mesial plate projects into it. 



The external area is loosely vesicular, without a well-defined 

 inner wall. 



Resemblances and differences. — The following horizontal 

 sections, figured by Thomson (Proc. Phil. Soc. Glasgow, vol. xiv, 

 1883), bear considerable resemblance to the Bristol form : — 



{Jlisiophyllum coniseptum (Keyserling), Thomson, pi. xiii, fig. 4. [In the 

 Explanation of the Plate, it is called Clisiophyllum Danai, Thomson.] 

 Here, however, only a small central portion of the mesial plate is specially 

 thickened; but axial symmetry is suggested. 



Histiophylhim Dicki, Thomson, pi. xii, fig. 6. 



Here, axial symmetry is suggested by the more prominent lamella, which 

 extends into the fossula, but there is no specially-thickened mesial plate crossing 

 the centre. [The genus Histiophyllum may be, perhaps, better placed in 

 Section III.] 



