BRITISH AVONIAN CONODONT FAUNAS 61 
the Middle Burlington Formation. In Western Europe it ranges from upper Cu II B 
into lower Cu II y. 
In West Germany Bischoff (1957) recorded the first appearance of Mestognathus 
beckmanni at the base of Cu III «, but Voges (1959) found a single specimen near the 
base of Cu II 6/y. Kronberg, Pilger, Scherp and Ziegler (1960) also found Mesto- 
gnathus beckmanmi in beds of Cu II 8/y age. Meischner (1962) in his chart of strati- 
graphic ranges (p. 31, fig. 10) showed the stratigraphic range of Mestognathus 
beckmanmi as commencing at the base of Cu II 8/7. The lowest occurrence of M. 
beckmanmi in the Avonian is in the upper part of the Laminosa Dolomite (the base of 
Sample C 15). Thus the upper part of the Laminosa Dolomite appears to be of 
Cu II 6/y age. 
In North America the base of the Bactrognathus-Taphrognathus Assemblage Zone 
is marked by the highest occurrence of Polygnathus communis, and the top by the 
lowest abundant occurrence of Gnathodus texanus, which makes its first appearance 
near the top of the zone. The youngest stratigraphic occurrence of Polygnathus 
communis in the Avonian is in the middle of the Laminosa Dolomite. It is con- 
sidered that the middle of the Laminosa Dolomite, above the youngest occurrence of 
Polygnathus communis (base Sample C 10), corresponds to the base of the Bactro- 
gnathus—T aphrognathus Assemblage Zone of North America. In North America this 
zone is confined to the upper part of the Burlington Formation and is correlated by 
Collinson, Scott & Rexroad (1962) with the upper part of the European Cu II 8/y 
Zone. Since Mestognathus has not been recorded from North America, our inter- 
mediate faunas provide important confirmation of this correlation. 
The Caninia Oolite in the Avon Gorge is characterized by the presence of two 
species, Polygnathus bischoffi sp. nov. and Mestognathus beckmanni. In Western 
Europe the youngest stratigraphic occurrence of Polygnathus bischoffi sp. nov. is at 
the top of Cull B/y. Gnathodus texanus s.s. occurs with Mestognathus beckmanni and 
Polygnathus bischoffi sp. nov. in the Caninia Oolite of Fall Bay, Gower. Since the 
youngest occurrence of Gnathodus texanus s.s. in North America is near the top of the 
Bactrognathus-T aphrognathus Assemblage Zone, the upper part of the Laminosa 
Dolomite and the Canimia Oolite are correlated with the Taphrognathus—Bactro- 
gnathus Assemblage Zone and with the upper part of the European Cu II 8/y Zones. 
A conodont fauna from Askeaton, Eire, contains specimens of G. texanus, P. 
triangulus pinnatus and G. girtyi. The specimens of G. girtyi are identical to those 
illustrated by Hass (1953) and clearly have developed from G. texanus. The 
Askeaton fauna is therefore younger than the Caninia Oolite fauna and is best 
correlated with the Caninia Dolomite. 
S and D Zones. 
The Cavusgnathus unicornis—A patognathus libratus and the Taphrognathus varians— 
Cavusgnathus—A patognathus Zones of the Avonian cannot be precisely correlated 
with the American assemblage zones. Although they have elements in common 
with both the Taphrognathus varians—A patognathus and the A patognathus? geminus— 
Cavusgnathus Assemblage Zones of the Mississippi Valley, there are also important 
