62 BRITISH AVONIAN CONODONT FAUNAS 
differences between the two faunal successions. The Taphrognathus varians— 
A patognathus Assemblage Zone includes the Warsaw, Salem and the lower part of the 
St. Louis Formations, and it is probably equivalent to the upper part of the Cu II 8 
Zone of Western Europe. The limits of this assemblage zone were defined by 
Collinson, Scott & Rexroad (1962) as follows : “‘ The lower limit is marked by the 
lowermost occurrence of Apatognathus? in the Valmeyeran Series plus the highest 
occurrence of common Taphrognathus varians. The upper limit is distinguished by 
the lowermost occurrence of Cavusgnathus and the youngest occurrence of Taphro- 
gnathus as well as by the lower limit of the upper zone of abundant Apatognathus”’. 
The lowest stratigraphic occurrence in the Avonian of specimens identical to the 
apatognathids illustrated by Rexroad & Collinson (1963) is near the base of Cj. 
Taphrognathus does not appear in the Avonian until the upper part of the S Zone, 
well above the first appearance of Cavusgnathus. Specimens transitional between 
Cavusgnathus and Taphrognathus, identical to those illustrated by Rexroad and 
Collinson from the boundary between the lower and upper St. Louis Formation, are 
found in Samples S 49 to S 58 of the Avonian. Cavusgnathus first appears in the 
Avonian in the middle of the Caninia Dolomite, well above the first appearance of 
Apatognathus, and well below the first appearance of Taphrognathus. In North 
America, however, the first appearance of Cavusgnathus is above the first appearance 
of Taphrognathus and A patognathus. 
In the Scottish conodont faunas, Taphrognathus varians occurs with Polygnathus 
lacinatus, a species which in the Avonian is common in the upper part of Zz and in the 
C Zone. Thus it would appear that the lowest occurrence of Taphrognathus in 
Britain may be considerably lower than that noted from the Avonian of the South 
West Province and it would then correspond more closely with the North American 
occurrence. 
The Apatognathus geminus—Cavusgnathus Assemblage Zone (Upper Se—D, of the 
Avon Gorge) is equivalent to the upper part of the St. Louis Formation of the 
Mississippi Valley and to the basal Cu III « of Germany. 
In Germany, Mestognathus beckmanni ranges from the base of Cu II @/y (and may 
extend into Cu II «) to the middle of Cu III y (Bischoff 1957) although Meischner 
(1962) gives the upper limit as Middle Cu III 8. Gnathodus bilineatus is present in 
the Cu III and E Zones of Germany and Britain (Collinson & Druce im press : Higgins 
1961), and G. girtyi girtyi ranges from Cu II y into the Namurian (Bischoff 1957), 
although Meischner (G. givtyi Form A = G. girtyi girtyt) restricts it to the Cu III « 
Zone, with the possibilities of homeomorphic development throughout Cu III. In 
view of Bouckaert & Higgins’ (1963) record of the species from the Eg, of the Belgian 
Dinant Basin and its presence in P; and Pg of the Yoredale succession, the German 
ranges cannot be used as the basis of correlation. The absence of the gnathodids, 
G. semiglaber and G. texanus, as well as species of the genus Polygnathus, from this 
Lower Dg Subzone indicates that it is younger than Cu II y. 
Meischner (1962 : 31) has shown that G. girtyi simplex (G. girtys Form B of 
Meischner) first occurs very near the base of Cu III 8, while G. homopunctatus 
occurs just below the base of Cu III 8. Thus the upper boundary of the Mestog- 
