BRITISH AVONIAN CONODONT FAUNAS 209 
In aboral view the basal cavity is large and occupies the anterior half of the 
platform ; the outer side of the margin has a slight fold. The basal cavities of right 
and left forms are bilaterally symmetrical elements. 
REMARKS. In the basal beds of the Z Zone specimens of P. primus occur, which 
resemble P. dentilineatus in outline, but which differ in detail because the nodes or 
ridges of the right side of the platform are not confined to the margin, as in P. 
dentilineatus, but are elongated and extend into the carina. Also in adult P. primus 
there is a greater number of ridges than in adult P. dentilineatus. The anterior 
trough on the left side of the platform is present in both P. dentilineatus and P. 
primus, but in the latter species the nodes at the posterior of the left side of the 
platform are extended to the carina and form ridges. In addition the basal cavity is 
as wide as the platform in P. dentilineatus, but less wide in P. primus. 
Higher in the section, forms resembling P. dentilineatus are found, but it is thought 
that they represent growth stages of other pseudopolygnathids. P. stviatus Mehl and 
Thomas, for example, is similar to P. dentilineatus but can be shown by ontogenetic 
studies to be a growth stage of P. multistriatus. 
We agree with Klapper (1966 : 15) that P. dentilineatus developed from double 
rowed forms of Spathognathodus. We do not regard S. costatus ultimus as their direct 
ancestor. 
Pseudopolygnathus expansus sp. nov. 
Plate 5, figs. 2a—c, 4a—c 
DERIVATION OF NAME. After the expanded anterior part of the platform. 
Diacnosis. Pseudopolygnathid with straight axis in the mid and anterior thirds, 
but curved in the posterior third. Anterior third of the right platform is expanded. 
Outer margin, except expanded part, has convex outline and is ornamented by nine 
or more transverse ridges. Anterior to mid length of unit, margin of inner side of 
platform is expanded for a short distance. Basal cavity is asymmetrically flared ; 
both inner and outer anterior margins of basal cavity expanded laterally, but that of 
outer margin is greater. 
MATERIAL. 3 specimens: Holotype X 483, Paratype X 482 (both figured). 
TYPE LOCALITY AND HORIZON. Avon Gorge, the middle of the K Zone. Sample 
K 12. 
RANGE. Avon Gorge, K 12. 
DEscRIPTION. The platform is lanceolate in outline. The axis of the unit is 
straight in the mid and anterior thirds, but curved in the posterior third. The 
platform is widest anterior to mid-length and is two to three times as long as wide. 
The convex margin of the outer side of the platform is interrupted by a lateral 
expansion of the platform, mid-way between the anterior edge and the mid-length of 
the unit. The outline of the inner margin posterior to the mid-length is straight to 
slightly convex. At the mid-length the inner platform is expanded and tapers 
gradually from the expanded portion to the anterior extremity. The outer side of 
the platform is ornamented by up to nine transverse ridges, extending from the 
