BRITISH AVONIAN CONODONT FAUNAS 245 
(e) Stratigraphic ranges 
The precise ranges of all conodont species recovered are shown by range charts 
(Figs. 49-58). The ranges of the more stratigraphically useful genera and species 
are described. Patrognathus gen. nov. is confined to the K Zone. Clydagnathus 
gen. nov. is found in the K and Lower Z Zones of the North Crop and Shropshire, but 
is rare in large faunas of the same age from the Avon Gorge. Szphonodella is very 
rare in the Avonian, being confined to the Upper K Zone (p. 32). 
Pseudopolygnathus extends from the basal K to the C; Zone. It is represented by 
numbers of short-lived species and is abundant in the Avon Gorge, but less common 
in the North Crop (p. 32). 
Gnathodus is a long-ranging Avonian genus, but individual species are valuable for 
correlation in the Z, C and D Zones (p. 34). Spathognathodus is equally long- 
ranging, but several short-lived species are valuable index fossils in various portions 
of the succession. 
Polygnathus ranges from the K to the top of the C; Zone, and is represented by a 
number of species. Mestognathus extends from the C; to the Deg Zone (p. 35), while 
Cavusgnathus is most characteristic of the Cz, S and D Zones. Taphrognathus is 
restricted to the Upper Sg Subzone (p. 35) in the south west, but occurs in the C 
Zone of Roxburghshire. 
Fourteen conodont assemblage zones are established 
(£) Geographic variation 
Our study shows several striking examples of what appear to be geographical 
differences in contemporaneous conodont faunas. These include representatives of 
the genera Szphonodella (p. 32), Pseudopolygnathus (p. 32), and Clydagnathus 
(p. 32) among others. These differences reflect variation not only between faunas 
separated by intercontinental distances, but also between faunas from the same 
general depositional basin (e.g. the Avon Gorge and the North Crop). The degree of 
such geographic variation is shown to be often greater than that generally admitted 
by most conodont workers, and is an important consideration in stratigraphic 
correlation. The absence of certain conodont genera and species from some areas 
may represent either the absence of one or more groups of conodont-bearing animals 
(conodontifers) of whatever taxonomic level, or the homoeomorphic replacement of 
certain conodonts within the same broad type of natural assemblage of the cono- 
dontifers. 
(g) Correlation within the British Avonian 
Avon Gorge—North Crop 
The broad equivalence of Vaughan’s coral—brachiopod zones as recognized in these 
two areas is supported by a comparison of the conodont faunas. The basal K strata 
of the North Crop are probably slightly younger than those of the Avon Gorge. The 
Upper K Zone of the Avon Gorge is equivalent to the uppermost K and basal Z Zones 
of the North Crop (p. 46). 
