BRITISH AVONIAN CONODONT FAUNAS 193 
the middle half, but the whole platform is sharply tapered and is pointed posteriorly. 
There is a conspicuous lack of general ornament, except for barely perceptible 
transverse ridges and nodes on the outer lateral faces, and a median carina, which is 
also very inconspicuous, consisting of low fused nodes, which runs posteriorly from 
the anterior blade but does not divide the platform into two equal halves. It runs 
into the posterior portion of the platform. The anterior inner edge is higher than 
any other part of the platform except the median outer lateral edge, and is 
ornamented by low fused nodes, up to six in number. 
In inner lateral view the “ injury ”’ to the inner lateral platform makes a prominent 
feature. The platform is deepest anteriorly and the “injury ”’ shows a secondary 
projection. 
In aboral view the individual shows a typical polygnathid form, with a minute and 
thickly-lipped cavity restricted to the anterior fifth of the platform, and extended 
anteriorly and posteriorly as a slit-like groove. The anterior blade is poorly 
preserved but it is relatively short, with a rounded anterior aboral margin, and has at 
least one conspicuous denticle near its posterior end. 
Polygnathus sp. 
Plate 31, fig. 21 
MATERIAL. 3 specimens: figured, X 130. 
Rance. Avon Gorge D 22. 
DESCRIPTION. Rexroad (1957 : 41) has noted the presence of small numbers of 
polygnathids in the Renault Formation at two localities. He has described three 
fragments, and very small numbers of specimens are also present in the D Zone of the 
Avon Gorge and the North Crop. In all three cases this represents an abnormally 
high stratigraphic occurrence of the genus. In the Mississippi Valley, for example, 
its generally accepted range extends upwards only into the Burlington Formation. 
A single specimen is illustrated. The anterior blade is broken, but this specimen 
displays strongly developed aboral lips with a conspicuous pit developed near the 
anterior end of the platform, and a well-developed slit-like basal cavity extending the 
whole length of the platform. 
Genus PRIONIODINA Ulrich & Bassler 1926 
1925 Prioniodina Bassler : 219 (nom. nud.). 
1926 Prioniodina Ulrich & Bassler: 17, 18. 
1934 Subbryantodus Branson & Mehl: 285. 
TYPE SPECIES. Prioniodina subcurvata Ulrich & Bassler 1926. 
In 1925 Bassler erected the generic name Prioniodina, and in 1926 with Ulrich as 
senior author, he gave the following description : “ Base of tooth more or less 
curved, crowned with numerous, sub-parallel, rounded, discrete denticles all inclined 
in one direction, one of which located in the median third, is considerably larger than 
the others. ”’ 
