84 THE AVONIAN OF THE AVON GORGE 
In fine, the zonal index merely connotes the whole faunal 
assemblage which is the ultimate expression of a biozone. 
The Zonal The faunal assemblages which characterize the suc- 
System cessive zones and subzones in the Carboniferous 
Limestone Series of the South-Western Province will be suffi- 
ciently understood from the fossil lists which are given under 
each division in the descriptive portion of this paper, and the 
zonal and subzonal indices are set forth in the same place. It 
is only necessary, here, to draw attention to the implication of 
certain new terms which are employed in the zonal scheme. 
Avonian denotes the whole interval of time during which the 
Carboniferous Limestone Series of the South-Western Province 
was being laid down. It is divided into two main periods, the 
Clevedonian and the Kidwellian, which are characterized by 
essentially distinct faunas. 
The Clevedonian is subdivided into three zones, namely the 
Cleistopora-Zone, the Zaphrentis-Zone and the Syringotkyris- 
Zone. 
The Kidwellian is divided into two zones, namely the Seminula- 
Zone and the Dibunophyllum-Zone. 
Each zone is again divided into subzones, and, for local use 
only, into Phases. 
The highest portion of one zone and the lowest portion of the 
next usually exhibit a mingling of the two faunas which are 
respectively characteristic of the two zones ; such levels of 
faunal overlap are termed Horizons. 
The use of a new term, Avonian, in place of the old and well- 
established Carboniferous Limestone, is a necessary outcome of 
the knowledge acquired by recent zonal work. A deposit is 
Lower Avonian, for example, because it contains a definite 
faunal assemblage ; it is Lower Carboniferous Limestone, merely 
because it happens to occur at the base of a local series of lime- 
stones. For example, Dr. Wheelton Hind has recently shown ^ 
1 Abstracts of Proc. Geol. Soc. (1906), p. 88. 
