THE AVONIAN OF THE AVON GORGE 
133 
This genus was founded in the belief that mural pores were 
absent, but good specimens demonstrate that this is an error ; 
consequently Beaumontia can only be regarded as a section of 
the genus Michelinia. 
Figures: Ed= and H. Plate 45 
Certain small forms of Beaumontia are difficult to separate 
from Favosites, and these occur, with the typical examples 
of Beaumontia^ at the very top of the Avonian. 
The continuous series : Favosites, Michelinia, Beaumontia 
and cf. Favosites, affords an excellent example of one of the 
simplest lines along which evolution works ; (1) stability and 
simplicity, (2) instability and exaggeration, (3) reversion to 
simplicity, (4) extinction. 
Cleistopora. 
Calyx view. 
Vertical section, showing the coral 
attached to a foreign body. 
The true relationship of this genus is extremely obscure. 
The coralium is a thick circular or oval plate whose upper 
surface is divided by low walls into not-numerous hexagonal 
calices ; the lower surface is fiat and attached to foreign 
bodies. 
The floor of the calyx is flat, but rough (as if laid with 
tiny cobbles). 
The calyx view of the coralium resembles that of Michelinia, 
but the walls and floor in Cleistopora are built up of a 
closely-packed mass of spongy tissue. 
