BRITISH AVONIAN CONODONT FAUNAS 185 
mid-length it is only slightly narrower than at the anterior. The platform is two and 
a half times as long as wide and tapers to the pointed posterior. The margins of the 
platform are slightly upturned in the anterior and mid-thirds, and the oral surface is 
ornamented by a number of transverse ridges, which are more strongly developed at 
the margin. The margins of the platform are equal in height to the carina and on 
either side of the carina there is a trough, which is more strongly developed at the 
anterior. The trough opens tothe anterior. The anterior blade is of the same length 
as the platform and consists of 6 denticles, which are highest at mid-length. The 
carina consists of nodes fused for their entire length and extends a short distance 
beyond the posterior extremity of the platform. The basal cavity; situated in the 
anterior third of the aboral surface, is small and rounded in outline, with thick lips. 
In some specimens a groove extends posteriorly from the basal cavity to the posterior 
extremity of the unit. 
REMARKS. Compared with Polygnathus inornatus inornatus the platform of P. 
bischoffi is more elongate, broader in the anterior half and tapering uniformly in the 
posterior half. The platform also has more convex margins and is arched in lateral 
view. Polygnathus bischoffi has less strongly developed anterior troughs than 
Polygnathus inornatus inornatus, because the margins of the platform in the anterior 
third are not as strongly upturned. 
Polygnathus bischoffi is characteristic of the C Zone both in the Bristol area and in 
other parts of the South West Province. It makes its first appearance near the base 
of the Laminosa Dolomites. Bischoff obtained the specimens of Polygnathus 
tnornatus which he illustrated from the Cu II Siphonodella Subzone. <A specimen, 
which Voges referred to as Polygnathus cf. flabellus, and which is here placed in 
synonomy, was found by him in the Siphonodella p. triangulus triangulus Zone, 
which is Upper Cu I in age. 
The “ Polygnathus inornata”’ group 
Rexroad & Scott (1964 : 35) remarked that “ P. inornata is a remarkably varied 
species”’. They included within their concept of this species P. sagittarius Young- 
quist & Patterson, a junior synonym of P. lacinatus Huddle which we regard as a 
separate and variable species in its own right. However, even within the much 
narrower terms of our present diagnosis, there is still a marked degree of variability. 
The transition between P. inornatus and P. lobatus was noted by Rexroad & Scott 
(1964 : 35), and can also be seen in our faunas. There is also a tendency for the 
development of a rostral ridge in both P. inornatus and P. lobatus. In a few cases, 
breakdown of the transverse platform ridges into transverse lines of nodes is also 
seen. The P. nornatus fauna occurs within the range of the genus S7phonodella and 
these morphological details are characteristic of that genus, which is extremely rare 
(0-5% of total conodont fauna) in our faunus. It seems that the P. inornatus group 
show morphological developments which carry it towards, but not as far as, the 
characteristic morphology of the genus Szphonodella. 
The P. inornatus s.s. forms with a well-developed rostral ridge are here given sub- 
