﻿286 DK. A, VAIJGHAN ON THE PAL^IONTOLOGICAL [May I905, 



Here I include Centrophyllum, Rhodopliyllum, Cymatiopliyllum, 

 and AlbeHia, all of them genera established by Thomson. 



It will be unnecessary, in this place, to discuss the value of the 

 distinctions which Thomson draws between the above genera, 

 since no examples have as yet been found, in the area with which 

 I am dealing, that can be definitely assigned to this section. 



Section IV. (The Aulophyllum-Group). 



Clisiophyllids, the central area of which is very sharply denned 

 by a circular wall, projecting, as a cusp, into the fossular gap. 

 The central area is composed of a network of vesicles, which are 

 very minute round the circumference, but larger in the centre. 

 The boss is a cratered cone. 



I have not seen sufficient material to judge of the value of the 

 separation of Cyclopliyllum, Dana & Thorns., from Auldphyllum, 

 Edwards & Haime. The only form with which I am personally 

 acquainted from this area, seems to be identical with Cyclopliyllum 

 pachyendotJiecum, Thomson, Proc. Phil. Soc. Glasgow, vol. xiv (1883) 

 p. 493 & pi. xiv, fig. 1. 



Section Y. (The Lonsdalia-Group). 



Clisiophyllids with a sharply bounded central area, a strong inner 

 wall, a narrow external area, and a strongly-developed peripheral 

 area, composed of large vesicles. 



This group probably includes Aoeophylliim as well as Lonsdalia 

 (see above, p. 281). 



Examples from the Bristol Area. 



Lonsdalia. 



Lonsdalia eloeifobmis (Elem.). 



As interpreted in Edwards & Haime, ' Monogr. Brit. Foss. Cor.' pt. iii 

 (1852) p. 205 & pi. xliii, figs. 1-2. 



A dendroid variant of this type, resembling, externally, Lonsdalia 

 rugosa, M'Coy, is very common in the Bristol area. 



Lonsdalia etjgosa, M'Coy, 'Brit. Palaaoz. Eoss.' 1851, p. 105 & 

 pi. 3b, figs. 6-6 c. 

 Typical examples are common in D. y of the Avon section. 



Notes on the Evolutionary History of the Clisiophyllids, 



as developed in the Bristol and Neighbouring 



Areas. 



The earliest occurrence of the Clisiophyllids is in the Syringo- 



thyris-Zone of the Mendips and Tenby. These forms probably belong 



to a single species, which exhibits the following characters : — 



(1) The tabula in the central space are vaulted up into tall, laterally- 



compressed cones, regularly radiated by lamellae, and more or less 

 conspicuously crested. 



(2) The primary septa are stout, and of nearly-uniform thickness throughout ; 



the secondary septa are short and stout. 



(3) A thick outer wall is formed by the thickening of the ends of the septa. 



