THE AVONIAN OF THE AVON GORGE 
141 
Lithostrotion, 
Corallum compound and either dendroid or massive. 
Septa alternate and attached to the outer wall ; uniform in 
thickness, without special thickening at any point of their 
length. 
The primary septa usually stop short of the centre. The 
secondary septa never project far beyond the inner wall. 
Septal break inconspicuous. 
An external area, radiated by both series of septa, is always 
developed but is very variable in width, being almost absent 
in Lith. junceum, and very broad in Lith. affine. 
The tabulae are broadly convex in the dendroid species, 
and strongly conical in the massive forms. 
A solid lath-shaped columella is always present, but its 
degree of development varies considerably, being greatest in 
those species which have strongly arched or conical tabulae, 
and least in those with flattened tabulae. 
Vesicles are practically confined to the external area and 
vary in number with its width. 
An inner wall is formed by the closer approximation of the 
vesicles at the inner boundary of the external area. 
The essential characters of the genus are : — 
(1) The lath-shaped columella. 
(2) The absence of plates starting from the columella and 
radiating outward (lamellse). 
Figures: Ed. and H. Plates 38 to 42 
Diphyphyllum. 
LithostrotionAike. Gyathophyllum-hk.e. 
