40 BRITISH AVONIAN CONODONT FAUNAS 
possible markers for zonal limits, but the first appearance of Gnathodus girtyi 
collinsont subsp. nov. was the one in our opinion with the most widespread and 
meaningful application. 
Within a broad depositional basin there is a general similarity between the species 
of “ bar and blade ”’ conodonts in each of the upper zones, but these similarities do 
not extend to more distant areas, and we have therefore not listed them as character- 
istic of the zones as a whole. 
(d) Intra-Avonian correlation in Britain 
(i) Avon Gorge—North Crop (Figs. 49-52). 
There is a close overall similarity between the Lower K conodont faunas of boththe 
Avon Gorge and the North Crop (Fig. 14). The lowest beds of both are characterized 
by the presence of Spathognathodus plumulus plumulus sp. et subsp. nov. and Patro- 
gnathus variabilis gen. et sp.nov. The absence in the lowest beds of the North Crop 
of Pseudopolygnathus vogest sp. nov. which is present in the basal beds of the Avon 
Gorge, probably implies a slightly older age for the North Crop strata. Siphonodella, 
a most important diagnostic genus for the Carboniferous, and the Polygnathus 
inornatus group first appear at comparable horizons in the North Crop and the Avon 
Gorge. The last appearance of Siphonodella in both sections coincides with the first 
appearance of Spathognathodus robustus, a species whose first appearance overlaps 
the last appearance of the Polygnathus inornatus group. 
In the Avon Gorge, Spathognathodus costatus costatus first appears just below the 
traditionally accepted K—Z Zone boundary, whereas in the North Crop, it occurs just 
above it. The general ranges of all these species are so similar in the two areas, that 
correlation is relatively straightforward. Details are given on p. 36 in the zonal 
discussion and in Fig. 14. 
In spite of the overall lithological similarity between the Lower Z Zone strata in 
both areas, there is a lack of any detailed resemblance in their platform conodont 
faunas. We tentatively interpret this as the result of geographical isolation of 
certain conodontifers (see p. 36). | This probably reflects the influence of palaeo- 
geographical factors, in spite of the fact that other studies have suggested a relatively 
uniform depositional basin in this area during early Z times. 
The incoming of Polygnathus lacinatus and of characteristic Pseudopolygnathus 
multistriatus in the higher part of the Z Zone of both areas provides a firm basis for 
correlation. The absence of Pseudopolygnathus cf. longiposticus in the North Crop 
suggests that the higher part of the Z Zone is unrepresented there. 
Se and C2S; rocks from the North Crop have yielded no conodonts, and Avon Gorge 
faunas of this general age and also of the D; Subzone are also sparse. The presence of 
Gnathodus bilineatus near the base of Dg in both areas re-establishes the correlation 
in the higher part of the Avonian. Above this level, conodonts are so rare in the 
Avon Gorge that no useful comparison can be made with the abundant faunas of 
the North Crop. 
