60 BRITISH AVONIAN CONODONT FAUNAS 
lation, in which the ranges of gnathodid species were compared, is also limited by a 
comparable, tantalizing “zone of few gnathodids”’ in the Middle and Upper 
Hannibal of North America, and if, as we believe, both correlations are equally 
reasonable, they are also equally vulnerable, because of a comparable lack of 
detailed phylogenetic development in Britain and the United States. 
The present available data are insufficient to allow us to make a final choice 
between the two alternative correlations ; although we have shown the second on 
our main correlation table (Fig. 12), we have summarized the evidence for both 
on Fig. 16). The total stratigraphic differences involved between them are not 
great, and it is perhaps a paradoxically satisfying aspect of the precision which we 
believe conodonts offer, that we should be dissatisfied with an uncertainty of some 
80 ft. in such a transatlantic correlation. 
A new conodont fauna associated with the goniatites Protocamites and Muen- 
steroceras from the Berwick Formation of Australia (Dr H. T. B. Jenkins, personal 
communication) contains a fauna which is similar to the Z Zone fauna of the Avonian 
in many respects. The pseudopolygnathids are identical to Z; and Lower Zz speci- 
mens. Associated with the pseudopolygnathids are many gnathodids, most of which 
are new species, although a few are similar to G. punctatus, a species characteristic of 
the upper part of Zz. Two specimens of Bactrognathus also occur in the Australian 
fauna, suggesting that Bactrognathus is older than has hitherto been thought likely. 
The fauna of Samples Z 31 to Z 38 with Gnathodus semiglaber and Pseudopoly- 
gnathus multistriatus is similar to that of the Gnathodus semiglaber—Pseudopoly- 
gnathus multistriatus Assemblage Zone of the Mississippi Valley. This Assemblage 
Zone coincides with the “ Sedalia ’’ Formation of the Mississippi Valley and has been 
correlated with the lowermost part of the Cu II 8 Zone of Western Europe. Dr W. 
Ziegler (personal communication) believes that the gnathodid fauna of Samples 
Z 34-Z 38 is identical to that found in Western Europe in beds equivalent to the 
Siphonodella crenulata Zone (Voges 1959) and thus of Cu II « age. 
C Zone. 
In the upper part of the Z Zone and in the lower part of the Laminosa Dolomite, 
Gnathodus antetexanus has been found, associated with Pseudopolygnathus cf. 
triangulus triangulus, Polygnathus communis and Polygnathus lacinatus. The lower 
limit of the Bactrognathus—Polygnathus communis Assemblage Zone in the Mississippi 
Valley coincides with the earliest abundance of Guathodus antetexanus, and the upper 
with the youngest stratigraphical occurrence of Polygnathus communis. In the 
Avonian of the Avon Gorge, the youngest stratigraphical occurrence of Polygnathus 
communis is in the middle of the Laminosa Dolomite (the base of Sample C Io). 
The Bactrognathus—Polygnathus communis Assemblage Zone is characterized by the 
presence of Polygnathus communis, Pseudopolygnathus multistriatus, Pseudopoly- 
gnathus triangulus pinnatus and by being the zone of abundant Gnathodus ante- 
texanus. Samples Z 38-C g are correlated with this Assemblage Zone. In North 
America, the zone extends from the base of the Fern Glen Formation to the top of 
