44 BRITISH AVONIAN CONODONT FAUNAS 
identical to those from the St. Louis illustrated by Rexroad & Collinson (1963), have 
been found in this assemblage zone. 
Apatognathus geminus—Cavusgnathus Assemblage Zone 
CHARACTERISTIC SPECIES : Cavusgnathus spp. and A patognathus geminus (Hinde). 
Limits : The lower limit is marked by the last appearance of Taphrognathus 
varians Branson & Mehl. The upper limit of the zone is marked by the first appear- 
ance of Gnathodus bilineatus (Roundy). 
REMARKS: This assemblage zone occupies the Upper Seg and the D; Subzones 
(Samples S 59-D 9). No D; Subzone conodonts were recovered from the North Crop. 
The assemblage zone is one of few conodonts, but is characterized by the presence of 
the genera A patognathus Branson & Mehl and Cavusgnathus Harris & Hollingsworth. 
It is tentatively correlated with the A patognathus geminus—Cavusgnathus Assemblage 
Zone of North America, which is found in the upper part of the St. Louis Formation. 
This is equivalent to the lowest part of Cu III « in Germany. 
Sample D 10 which marks the first appearance of G. bilineatus (Roundy) is tenta- 
tively taken as the basal sample of Dz although the D;—D2 boundary in the type 
section is difficult to determine. One anomalous feature of this zone is the occurrence 
of a fauna from the North of England associated with Bollandoceras hodderense (Bj). 
This would generally be correlated with Sg of the Avonian (e.g. Thomas & Prentice 
1965 : 43). This fauna includes M. bipluti Higgins with G. symmutatus sp. nov., 
G. girtyt simplex Dunn, G. commutatus Branson & Mehl), G. bilineatus (Roundy) and 
G. homopunctatus Ziegler. 
The subsequent assemblage zones were established from the Dz and Dg successions 
on the North Crop. The few scattered conodonts of this age that were recovered 
from the Avon Gorge show a general similarity to the North Crop faunas. 
Mestognathus beckmanni—Gnathodus bilineatus Assemblage Zone 
CHARACTERISTIC SPECIES: Mestognathus beckmanni Bischoff, Cavusgnathus 
unicorms Youngquist & Miller, Spathognathodus scitulus (Hinde), Apatognathus 
bladus sp. nov., Hibbardella abnormis Branson & Mehl, Neoprioniodus montanaensis 
(Scott), Gnathodus girtyi girtyt Hass, G. bilineatus (Roundy), and indeterminate 
magnilaterellids. 
Limits : The lower limit of the zone is tentatively taken as the first appearance 
of G. bilineatus (Roundy), but the absence of conodonts in samples of D; North Crop 
strata makes this position provisional. Further collecting will probably reveal an 
earlier occurrence of this species, which is present in Cu III « of Germany. The top 
of the zone is marked by the incoming of G. mononodosus sp. nov. 
REMARKS : This assemblage zone is represented by the lowest 60 ft. of the De 
Subzone of the North Crop (Samples CYD 1-CYD 6). It is equivalent to part of the 
Cu III « in Germany, and broadly equivalent to the St. Genevieve Formation of the 
Mississippi Valley. The zone is no younger than the Pj, Subzone of Northern 
England. 
