BRITISH AVONIAN CONODONT FAUNAS 35 
inornatus vexatus subsp. nov. and P. lobatus inflexus subsp. nov. appear in the Middle 
K Zone. The P. inornatus group becomes extinct in the upper part of the K Zone. 
The P. lacinatus Huddle group appears in the uppermost Z Zone, together with P. 
lacinatus asymmetricus subsp. nov. The C Zone is characterized by P. lacinatus s.s. 
and P. bischoffi sp. nov. 
The genus Mestognathus first appears in the upper part of the Cy Subzone, where it 
is represented by M. beckmanni Bischoff, which extends into the D Zone. M. bipluti 
Higgins first appears in the D; Subzone and extends into the higher part of Dz, where 
M. neddensis sp. nov. is also present. Neither extends into the Dg Subzone. 
Cavusgnathus is most characteristic of the Cz, S and D Zones. C. unicornis 
Youngquist & Miller is present in the Cz Caninia Dolomite and extends into the D 
Zone. C. charactus Rexroad first occurs in the C2S; Zone, and C. convexus Rexroad 
is characteristic of the Middle S Zone. 
The genus Taphrognathus is restricted to the Upper Sz Subzone, where it is 
represented by a single species, T. vartans Branson & Mehl. 
The striking genera Bactrognathus, Doliognathus, Scaliognathus and Staurognathus, 
which are distinctive components of faunas of comparable age in other areas, are 
unrepresented in our Avonian faunas, in which the genus Elictognathus is represented 
by only a single fractured specimen. It seems probable that these genera were 
geographically or ecologically restricted. 
Most of the genera of “‘ bars and blades” have a long stratigraphic distribution, 
but a number of species have proved valuable in local correlation. Ligonodina beata 
nom. nov., Hindeodella corpulenta Branson & Mehl, and H. subtilis Ulrich & Bassler 
are common Upper K and Z Zone forms. Apatognathus makes its first appearance in 
the Zz Subzone. A. geminus (Hinde) is present in the C; Subzone and extends into 
the D Zone. A. scalenus Varker appears in the Upper C; Subzone. A. bladus sp. 
nov. is confined to the Dz Subzone. Prioniodina laevipostica (Rexroad & Collinson) 
is limited to the Upper Dz Subzone. Neoprioniodus scitulus (Branson & Mehl), N. 
peracutus (Hinde), Hindeodella undata Branson & Mehl, and H. antecomplex Collinson 
& Druce first appear in the Lower Dg Subzone. Magnilaterella clarkei sp. nov. is 
confined to the D Zone, as is Kladognathus. 
The precise ranges of these and other species are given on Figs. 49-58. The low 
yield of conodonts from many samples makes reliance upon any single “‘ index fossil ”’ 
an unreliable method of correlation, but the general characteristics of the individual 
conodont assemblages are distinctive for each of our main stratigraphic divisions, 
and these provide the basis of a relatively precise scheme of correlation. 
(c) Avonian Conodont Biostratigraphical Zones 
A series of 14 conodont assemblage zones has been established. We have used the 
Avon Gorge and the North Crop successions as our type sections, using the latter to 
provide the three highest Avonian Zones, which are largely represented by non- 
_ carbonate sediments in the Avon Gorge area. We have therefore included in this 
section a detailed discussion of the correlation between these two areas, in order to 
provide a basis for the zonal scheme, which we have used (p. 46) to establish correla- 
