64 BRITISH AVONIAN CONODONT FAUNAS 
occurrence : Gnathodus commutatus, Gnathodus delicatus, Gnathodus semiglaber and 
Polygnathus communis. Stphonodella obsoleta and Siphonodella duplicata are also 
present. 
In the Avonian Gnathodus delicatus first appears in Z;. It is followed in the Lower 
Zz Subzone by Gnathodus simplicatus and in the upper part of Zz by Gnathodus semi- 
glaber. Thus the upper part of Z; and the Zz Subzone would appear to be broadly 
equivalent in age to Tng,. There are, however, two peculiarities in the Franco- 
Belgian Tng, conodont fauna: firstly, the late first occurrence of Polygnathus 
communis, and secondly, the occurrence of Pseudopolygnathus triangulus inaequalis. 
Tng» is characterized in the Franco-Belgian Province by the presence of Gnathodus 
delicatus, Polygnathus communis, Doliognathus excavatus, Scaliognathus anchoralis, 
Hindeodella segaformis, Gnathodus bilineatus, Gnathodus homopunctatus and Gnathodus 
girtyt. 
In Germany two of these species, Scaliognathus anchoralis and Hindeodella sega- 
formis, and also one of the genera, Doliognathus, are confined to the Scaliognathus 
anchoralis Zone (Cu II 8/y). Gnathodus delicatus first appears at the base of the 
anchoralis Zone. The youngest occurrence of Polygnathus communis is within the 
anchoralis Zone. Gnathodus homopunctatus is present at the top of the zone, as too is 
Gnathodus girtyt. 
Gnathodus bilineatus in Germany does not appear until after the youngest strati- 
graphic occurrence of Scaliognathus anchoralis (the anchoralis—bilineatus interval). 
In the Franco-Belgian Province, on the other hand, the first appearance of the two 
species is contemporaneous. 
In the Franco-Belgian Province the youngest stratigraphic occurrence of the genus 
Siphonodella coincides with the oldest stratigraphic occurrence of the genus Scalio- 
gnathus at the base of Tng,. In Germany Siphonodella extends into Cu III « in the 
Hartz Mountains of the Sauerland (Voges 1959). It extends into the same zone in 
Spain (Dr W. Ziegler, personal communication), but in North Africa (Dr G. Bischoff, 
personal communication) it is limited to Cu Il. The Genus Elictognathus became 
extinct a short distance beneath the anchoralis Zone, at the base of the upper 
Siphonodella crenulata Zone (Middle Cu II «), whereas, in the Franco-Belgian 
Province, the latter genus became extinct at the base of Tng,. Dr S.C. Matthews 
is at present investigating the distribution of the anchoralis Zone fauna in Europe. 
In North America the base of the Sedalia Formation is an unconformity and the 
genera Elictognathus and Siphonodella have their youngest stratigraphic occurrence 
at the top of the Upper Chouteau Formation. Scaliognathus anchoralis first appears 
at the base of the Sedalia Formation. At the base of the Fern Glen Formation the 
genera Doliognathus, Staurognathus and Bactorgnathus first appear. Gnathodus 
delicatus, Gnathodus cf. girtyit, Gnathodus semiglaber, Gnathodus cuneiformis and 
Gnathodus antetexanus also are present above the base of the Sedalia Formation. 
In Germany there is a gap in the known conodont sequence beneath the anchoralis 
Zone. It appears likely, however, that Tng, of the Franco-Belgian succession is 
equivalent in age to the German Cu II 8/y anchoralis Zone and the Cu II 8 anchoralis— 
bilineatus interval combined. It also correlates with the Fern Glen and Burlington 
