42 BRITISH AVONIAN CONODONT FAUNAS 
limit is marked by the disappearance of P. cf. longiposticus (Branson & Mehl) and by 
the replacement of G. semiglaber (Bischoff) by G. antetexanus Rexroad & Scott in the 
Avon Gorge. 
REMARKS: This assemblage zone occupies the uppermost part of the Z Zone in 
the Avon Gorge (samples Z 33-Z 38). It is absent, owing to an unconformity, in the 
North Crop. It is correlated with the Fern Glen Formation of North America, the 
Middle Cu II 8-y of Germany, and the Tnz, in Belgium. 
Gnathodus antetexanus—Polygnathus lacinatus Assemblage Zone 
CHARACTERISTIC SPECIES: Polygnathus lacinatus s.s. Huddle, Apatognathus 
geminus (Hinde), Apatognathus scalenus (Varker), Apatognathus petilus Varker, S. cf. 
cristulus Youngquist & Miller and G. antetexanus Rexroad and Scott. 
Limits : The lower limit of the zone is marked by the youngest occurrence of P. 
cf. longiposticus (Branson & Mehl). The lower limit of the genus A patognathus in the 
Avon Gorge is near the lower limit of the zone. The upper limit of the zone is 
marked by the oldest stratigraphic occurrence of Mestognathus beckmannt Bischoff. 
REMARKS : This zone occupies the lower part of the C; Subzone in the Avon Gorge 
(Samples C 1-C 14). It is not present on the North Crop. It is correlated with the 
lower, middle and lower upper parts of the Burlington Formation of North America, 
with the upper part of Tng, and possibly the lowest Tn, of Belgium and with the 
Upper Cu II B-y of Germany. 
In this assemblage zone conodonts are sparse. The genus Pseudopolygnathus is 
represented by a considerably smaller number of specimens than in the underlying 
zone. Specimens of the P. lacinatus group, S. cf. cristulus Youngquist & Miller and 
A. scalenus (Varker) dominate the fauna. The youngest stratigraphic occurrence of 
P. communis communis Branson & Mehl in the Avon Gorge is within this zone. 
Mestognathus beckmanni—Polygnathus bischoffi Assemblage Zone 
CHARACTERISTIC SPECIES: Mestognathus beckmanni Bischoff, S. cf. cristulus 
Youngquist & Miller, Polygnathus bischoffi sp. nov., A. scalenus Varker, A. petilus 
Varker, and A. geminus (Hinde). 
Limits: The lower limit of the zone is defined by the oldest stratigraphic 
occurrence of Mestognathus beckmannmi. The upper limit probably coincides with 
the zone of maximum abundance of G. texanus (Roundy). 
REMARKS :_ This assemblage zone occupies the upper part of the Cy Subzone in the 
Avon Gorge (Samples C 15-C 25). It is correlated with part of the Upper Burlington 
Formation of North America, with the lowest Cu II 5 of Germany and with part of 
the Tn3. of Belgium. 
The upper limit of this assemblage zone cannot be defined in the Avon Gorge, 
because the lower beds of the Caninia Dolomite do not contain conodonts. 
The conodont fauna obtained from the Upper Avonian Cz, S;, Sz, and D Zones of 
the Avon Gorge is sparse in comparison with that obtained from the Lower Avonian. 
