VAUGHAN : A NOTE ON CARBONIFEROUS SEQUENCE. 77 
The Dibunophyllum-Zone is approximately equivalent to the 
range of Dihunofhyllum, the typical genus of the ClisiophyUidan 
Corals. (The ClisiophyUidan plan of structure, indeed, affected 
the gentes of cognate famihes, as well as the group of which 
it is the essential property, and this fashion may be regarded 
as a time-trait distinctive of the Dibunophyllum-Zone.) 
The Dibunophyllum-Zone roughly coincides also with, the 
range of Productu^ ' giganteus' for, in the South- Western Province, 
P. ' giganteus ' comes in at the top of the Seminula-Zone, ranges 
to the top of D^, and becomes practically extinct in D^. 
There is, indeed, a striking paralleHsm between the Chsio- 
phyUidan Corals and that group of Producti which is loosely 
labelled P. ' giganteus.' Both are examples of the structural 
aggrandisement which precedes extinction. In Productus 
' giganteus,' the great size and mass of the shell, the irregu- 
larity of the ribbing (here confluent and flexuous, there forked 
and knotted) and the irregularly dispersed spines constitute 
a lavish excess of ornament which, almost invariably, heralds 
extinction. In the ChsiophyUids, the extraordinary com- 
plexity of the central area and the irregular vesicular structure 
of the periphery are highlj^ specialised characters, the acquisition 
of which necessarily involves loss of plasticity in the gens. 
Hence it is quite correct, both theoretically and practically, 
to select Productus ' giganteus ' as a zonal index, and this is the 
method adopted by the Belgian geologists.* 
The main objection to this course is that it is extremely 
difficult to detect the imderlying progressive variation in the 
case of a group so irresponsible and protean as that of Productus 
' giganteus.' (It seems to be the fact, however, at least in the 
South-Western Province, that forms of P. ' giganteus ' which 
converge towards P. latissimus distinguish rather than 
and, in the Xorth England Province, Professor E. J. Garwood 
has pointed out that P. latissimus marks a definite level throughout 
the area.) 
* In the " Legende de la Carte Geologique de la Belgique" (1900), the 
upper Visean (V2) is stated to be characterised by Productus Cora and 
P. giganteus. 
