32 BRITISH AVONIAN CONODONT FAUNAS 
ary dolomitisation, or possibly an unfavourable depositional environment, while the 
upper C; Caninia Oolite had an average yield of only 1 conodont per kilogram (range 
o-3 per kg.). Other conodont students have commonly found lithologically similar 
oolites to have low yields, and this may be the result of relatively rapid deposition. 
Of the 23 samples of the Caninia Dolomite which were processed, only one, a shelly 
calcarenite, yielded conodonts. 
S Zone samples had a yield of less than 1 per kilogram, and almost three quarters 
of the 72 samples processed were barren. Only 5 samples yielded an average of more 
than one conodont per kilogram. The Zone is marked by relatively little change in 
the conodont faunas, and, like the low yielding (1 per kg.) D Zone beds, probably 
represents rapidly deposited sediments. 
Broadly comparable variations in abundance are seen in strata of similar litho- 
logies from other sections. In the D Zone of South Wales, for example, the mean 
yield was 29 conodonts per kilogram of limestone dissolved, with a range from 0-198. 
In the Z Zone of the North Crop, the mean yield was g conodonts per kilogram, with 
a range from o-60. In general the lowest yields were those of the saccharoidal 
dolomites, and the highest were those of the basal beds of oolites. 
(b) Stratigraphical distribution of conodont faunas 
The precise ranges of all conodont species recovered for each of the various areas 
are shown on Figs. 49-58. The present section provides only a general view of 
the distribution of some of the more useful genera and species. 
Patrognathus gen. nov. is restricted to the K Zone. The genus Clydagnathus gen. 
nov. is found in the K Zone and in the lower part of the Z Zone of the North Crop and 
in Shropshire. It is rare in large faunas of the same age from the Avon Gorge, 
although it occurs abundantly in some other countries, e.g. Australia. The lowest 
occurrence of the genus is represented by the species C. gilwernensis gen. et sp. nov. 
and C. cavusformis gen. et sp. nov., which are rapidly replaced by Clydagnathus sp. A 
gen. et sp. nov. The Lower Z Zone species are C. unicornis gen. et sp. nov. and C. 
darensis gen. et sp. nov. The genus Siphonodella, which has proved to be abundant 
and of great stratigraphical value in other areas, is rare in the Avonian. It is con- 
fined to the upper part of the K Zone in both the Avon Gorge and the North Crop. 
The limited stratigraphic range of the genus in the Avonian is also reflected in an 
absence of the sequence of species, which has been described from the Mississippi 
Valley. 
The genus Pseudopolygnathus is one of the most distinctive components of the 
faunas of the K and Z Zones, extending from the base of the K Zone into the C 
Laminosa Dolomite. It is represented by a considerable number of species, most of 
which have restricted stratigraphical ranges. Pseudopolygnathus vogesi sp. nov. and 
Pseudopolygnathus expansus sp. nov. are confined to the lowest part of the K Zone, 
where they are distinctive species. The Z Zone is characterized by the incoming of 
abundant pseudopolygnathids, referable to the species P. primus Branson & Mehl 
and P. cf. dentilineatus E. R. Branson. These are replaced vertically by P. postino- 
dosus sp. nov. and P. nodomarginatus E. R. Branson. In contrast to the abundance 
