THE AVONIAN OF THE AVON GORGE 
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This genus is closely related to the preceding, of which it is, 
in fact, a more specialized representative. The essential differ- 
ence lies in the structure of the outer areas. 
In the external area, the vesicles are very densely packed, 
and the septal prolongations gradually thin out toward the 
outer wall. A peripheral area is developed in which the 
vesicles lie close together in parallel festoons. 
This genus is characteristic of D2 and, in its general type of 
structure, exhibits convergence with Lonsdalia. 
Dibunophyllum. 
Early and simple type. Late and specialized type. 
Central area bisected by a long mesial plate from which 
well-spaced, strong lamellae radiate outward. 
In the early forms, which are especially characteristic of 
Di, interseptal vesicles are loosely-packed and an inner wall is 
very indistinctly developed. 
In the later forms, characteristic of D2, the structure is 
highly specialized ; the external area is well developed, and 
there is usually a narrow peripheral area ; the central area is 
