142 
THE AVONIAN OF THE AVON OOROE 
This genus, as originally defined, includes only Lithostrotion- 
like corals in which there is no trace of a columella, and in 
which the tabulse are broad and almost flat. 
The septa are Avell spaced, and the secondary series is rela- 
tively short. 
Thus restricted, Difliyphyllum includes those Lithostrotions 
in which the columella is apparently never developed, but, 
since the degree of development of this structure in typical 
examples of Lithostrotion is known to vary with the elevation 
of the tabulae, it is clear that the distinction is not a natural 
one. The genus may, however, be usefully retained as a 
descriptive term, and it is convenient to extend its connotation 
so as to include Cyathophyllum-like forms with close-set septa 
and broad external area, in which the primary septa do not 
extend to the centre. The Di'pJiy'phyllum cited among the 
Di corals belongs to this section. 
Petalaxis, 
Petalaxis PortlocM. 
Corallum compound and massive ; built up of narrow 
prismatic corallites, which are easily separable since the walls 
of the separate corallites are perfectly distinct. 
Septa few and flexuous. 
Tabulse conical and well spaced. 
Columella usually conspicuous, but often apparently unde- 
veloped, its place being taken by the inner end of a long 
septum. 
Vesicular tissue very sparingly developed. 
I have* considerable doubt of the validity of this genus, 
since I am convinced that the specimen figured by Edwards 
