40 BRITISH AVONIAN CONODONT FAUNAS 
Branson & Mehl, Hindeodella subtilis Ulrich & Bassler. In the Avon Gorge : 
Pseudopolygnathus primus Branson & Mehl, Pseudopolygnathus cf. dentilineatus E. R. 
Branson, Pseudopolygnathus postinodosus sp. nov. On the North Crop: Clyda- 
gnathus unicornis gen. et sp. nov., and Gnathodus simplicatus sp. nov. 
Limits: The base of this assemblage zone is marked by the first occurrence of S. 
costatus costatus (E. R. Branson) and S. cf. cristulus Youngquist & Miller. The top of 
the assemblage zone is recognized by the incoming of abundant Pseudopolygnathus 
multistriatus Mehl & Thomas and by the replacement of G. delicatus Branson & Mehl 
by G. semiglaber (Bischoff) and G. antetexanus Rexroad & Scott. 
REMARKS : This zone occupies the uppermost K, the Z; and the lower part of the 
Zz Subzones in the Avon Gorge and most of the Z Zone on the North Crop. (Avon 
Gorge Samples K 18-Z 25 : North Crop Samples ZLA 2-ZLA 28). It is correlated 
with the uppermost Hannibal, the Lower and Upper Chouteau Formation of North 
America, with the Middle and Upper Cu II « of Germany, and with the uppermost 
Tng, and the Tng, of Belgium. 
This assemblage zone shows some geographical differences. Spathognathodus 
costatus costatus (E. R. Branson) first appears in the North Crop at the base of Sample 
ZLA 2, some distance within the Z Zone, and is quickly followed by S. costatus 
sulciferus (Branson & Mehl). In the Avon Gorge, on the other hand, these two 
species are present in the uppermost beds of the K Zone. It is believed that Pseudo- 
polygnathus primus Branson & Mehl evolved from S. costatus sulciferus Branson & 
Mehl. In the Avon Gorge at the base of the Z Zone there is a burst of pseudopoly- 
gnathids and the genus Pseudopolygnathus dominates the early Z Zone fauna. The 
genus Pseudopolygnathus is not present in the Lower Z Zone in the North Crop, its 
position in the natural conodont assemblages of the Z Zone on the North Crop being 
represented by other form genera. 
As a result, the following three subzones are recognized on the North Crop : 
C. Clydagnathus unicornis (Samples ZL 11-ZLA 28). 
B. Gnathodus simplicatus (Samples ZLA 15-ZL Io). 
A. Spathognathodus costatus costatus-Spathognathodus costatus sulciferus 
(Samples ZLA 2-ZLA 14). 
The base of the Spathognathodus costatus costatus-Spathognathodus costatus 
sulciferus Assemblage Subzone is marked by the first occurrence of S. costatus 
costatus (E. R. Branson) and S. cf. cristulus Youngquist & Miller. The subzone is 
recognized by the presence of the above, together with S. costatus sulciferus (Branson 
& Mehl), Gnathodus delicatus Branson & Mehl, and Apatognathus varians Branson & 
Mehl. The Guathodus simplicatus sp. nov. Assemblage Subzone is recognized by the 
first occurrence of Gnathodus simplicatus sp. nov. and the presence of Gnathodus sp. B. 
The top of this assemblage subzone is marked by the final occurrence of P. communis 
Branson & Mehl, N. barbatus (Branson & Mehl), S. costatus sulciferus (Branson & 
Mehl) and A. varians (Branson & Mehl). The C. unicorns gen. et sp. nov. 
Assemblage Subzone is marked by the paucity of the conodont fauna. What 
fauna there is, is dominated by the presence of C. unicornis gen. et sp. nov., which up 
to this point has been extremely rare. The top of this subzone is marked by the 
