36 BRITISH AVONIAN CONODONT FAUNAS 
tions between these type sections and Avonian sequences in Central England and 
Scotland. We believe that these conodont zones provide a useful method of 
correlation throughout the British Avonian, and that they offer a considerably 
higher degree of precision than existing coral-brachiopod zones. 
They show no detailed correspondence to the conodont zones established in either 
the Mississippi Valley or in West Germany, although the general faunal sequence in 
the three areas has enough broad resemblances to allow us to correlate between them 
with some confidence. 
The sequence and relationship of the 14 zones are shown in Fig. 12. They 
are defined and described below, and the detailed correlation between the Avon 
Gorge and the North Crop is discussed. The intercontinental correlations given for 
each zone below are generalized and approximate. There is a full discussion of 
correlation on p. 52. 
Patrognathus variabilis—Spathognathodus plumulus Assemblage Zone 
CHARACTERISTIC SPECIES: Patrognathus variabilis gen. et sp. nov., Spatho- 
gnathodus plumulus plumulus sp. nov., Pseudopolygnathus vogest sp. nov., Spatho- 
gnathodus plumulus shirleyae sp. et subsp. nov. and, in North Crop, Clydagnathus 
gilwernensis gen. et sp. nov. 
Limits :_ The base of the assemblage zone is not identified, but probably corres- 
ponds to the first appearance of P. variabilis gen. et sp. nov. The upper limit 
coincides with the oldest stratigraphic occurrence of Polygnathus inornatus inornatus 
Branson & Mehl, Polygnathus lobatus lobatus Branson & Mehl, and also of the genus 
Siphonodella. 
REMARKS : This zone occupies the lower and middle of the K Zone (Samples K 1— 
K ir in the Avon Gorge and Samples KL 1-KL 13 on the North Crop). It is 
correlated with part of the Cu I of West Germany, the lower and middle parts of the 
Hannibal Formation of North America and with the Tng, and Lower Tne, of Belgium. 
The lowest Lower Limestone Shale, which immediately overlies the Old Red 
Sandstone in the Avon Gorge, does not contain conodonts. The basal limestone 
stratum (Sample K 3) is characterized by the presence of S. plumulus plumulus 
subsp. nov. and P. variabilis gen. et sp. nov. Sample K 4 is the oldest from the 
Avon Gorge to contain pseudopolygnathids, whereas these pseudopolygnathids 
occur in the North Crop in the basal beds of the Lower Limestone Shale which 
directly overlies the Old Red Sandstone. 
Fic. 13. Chart to show the characteristic species of the conodont zones proposed in the 
present paper. Where a species is shown breaking the boundary between two successive 
zones, it is present in both. 
Patrognathus variabilis—Spathognathodus plumulus Assemblage Zone. 1a. Patrognathus 
variabilis gen. et sp. nov.—oral view. 1b. Patrognathus variabilis gen. et sp. nov.— 
lateral view. 2a. Spathognathodus plumulus plumulus sp. nov.—lateral view. 2b. 
Spathognathodus plumulus plumulus sp. nov.—oral view. 3a. Pseudopolygnathus 
vogest sp. noy.—oral view. 3b. Pseudopolygnathus vogest sp. nov.—aboral view. 
4a. Spathognathodus plumulus shirleyae subsp. noy.—lateral view. 4b. Spathognathodus 
