THE AVONIAH OF THE AVON OORGE 
143 
and Haime under the name Petalaxis Portlochi (PL 38, Figs. 4, 
4a) is merely a form of Lithostrotion irregulare in which the 
corallites have become prismatic by pressure (a condition of 
very common occurrence). 
In this paper I have retained the generic name for certain 
corals which differ markedly from the normal types of Litho- 
strotion in their spidery septa, as well as in the absence of a 
columella in many of the corallites. 
The only species is Petalaxis Portlocki, Ed. and H., emend. 
Vaughan, from the upper Dibunophyllum- zone. 
The CLISIOPHYLLIDS. 
All the members of this group are conical or cylindro-conical. 
Lonsdalia is invariably compound, and Koninckophyllum is not 
infrequently so. The remaining genera described below are 
almost invariably simple, although compound forms have been 
occasionally met with in all. 
No coral of this group can be definitely named except by the 
careful examination of a horizontal section and, consequently, 
the descriptions which follow deal mainly with the structures 
which are exhibited by such sections. 
The essential characters of this group are : — 
(1) The vaulting of the tabulae in the. central area. 
(2) The development of ‘ lamellae,’ a special series of plates 
which are attached to the upper surface of the arched tabulae, 
and which run, radially or spirally, down the sides of the tabular 
vault. V 
The central area, in a horizontal section, exhibits, in conse- 
quence, a very characteristic spider-web structure which is pro- 
duced by the intersection of the plane of section with the arched 
tabulae and with the radiating lamellae. 
The medial area is radiated by the ring of primary septa, the 
interspaces between which are occupied by large vesicles. 
The external area is radiated by the thin prolongations of the 
primary septa, and also by an equally thin intermediate series 
