﻿Vol. 64.] CARBONIFEROUS LIMESTONE OF THE MIDLAND AREA. 75 



well-developed, and shows comparatively few radial lamellae in the 

 central area. 



Comparison with the typical Dihunopliylla of the 

 South- Western Province. — As compared with Dibunophyl- 

 lumd,^ D. matlocJcense possesses a larger, more clearly- bounded 

 central area, a more specialized septal system, and a rather more 

 clearly-differentiated external area. 



Dihunophyllum xj,,^ which is more widely conical in form than 

 D. matlocl-ense, further differs from the Midland species in the 

 following characters, as seen in horizontal section : — The central 

 area is more strongly bounded, and the vertical lamellse, which 

 are more numerous, are usually curved ; while the mesial plate, 

 which is markedly thickened in the centre of the area, almost 

 invariably falls short of the circumference of the area, on the 

 antifossular side, by a considerable distance. The external area 

 is relatively broader, and is more closely vesicular, and the inner 

 wall is strongly defined. The septa not infrequently stop short of 

 the outer wall. 



DiBUNOPHTLLUM DEEBIENSE, Sp. UOV. (Pi. I, fig, 1.) 



Description. — The form is elongate-conical and large, some 

 specimens attaining a diameter of 2 inches. 



In a horizontal section : — The central area appears to be 

 completely bisected by a tiexuous mesial plate, which is thickened 

 centrally, and continued ijito the septal fossula as a thin lamella. 

 The radial lines, representing the cut edges of the vertical lamellae, 

 are irregularly and unequally developed. 



The primary septa, which are closely set (there are 72 of them 

 in the figured section, which is about 2 inches in diameter), are 

 appreciably thickened in the medial area. They taper outwards in 

 the external area, but extend to the outer wall, and are for the most 

 part straight throughout their length. The rudimentary secondary 

 septa have a very inconstant development. The septal fossula is 

 clearly marked. 



The broad external area, approximately equal in width to half 

 the radius of the coral, is closely vesicular, the dissepiments being 

 irregularly and very closely packed near its inner boundary. A 

 well-defined inner wall is formed by the thickening of the inner- 

 most rows of dissepiments. 



In the medial area, rectangular dissepiments, representing the 

 cut edges of the outward prolongations of the tabulae, are 

 numerous. 



Dihunopliyllum derhiense is a highly-distinctive form, differing 

 widely from any previously-described species with which I am 

 acquainted. It occurs rarely in the Lonsdalia-suhzone of the 

 Midland area. 



1 A. Vaughan, Quart. Journ. Geol. Soe. vol. Ixi (1905) p. 283 & pi. xxiv, fig. 1. 

 •^ Id. ibid. p. 284 & pi. xxiv, figs. 2 & 2 a. 



