204 BRITISH AVONIAN CONODONT FAUNAS 
gnathus foliaceaus and Pseudopolygnathus apetodus’. They also closely resemble P. 
primus E. R. Branson, which occurs in the Chappel Limestone of Texas (Hass 1959). 
The starting point for the P. primus development in the Avonian are specimens 
here referred to as Spathognathodus costatus costatus, which are found in the upper 
part of the K Zone and the base of the Z Zone. Forms with a row of lateral denticles, 
here referred to as Spathognathodus costatus costatus E. R. Branson, (they are not 
identical to Spathognathodus bischoffi sp. nov., but are homoeomorphs of the latter 
species) develop a ridge on the outer side of the left forms and on the inner side of the 
right forms. The edge of the ridge becomes crenulate and, at a later stage, nodes 
develop at the posterior extremity. The next stage is marked by an increase in the 
number of nodes along the ridge towards the anterior. The anterior extremity, on 
the outer side of the left forms and on the inner side of the right forms, extends as far 
anteriorly, as it does on the inner side of left forms and on the outer side of right 
forms. A small trough is present between the carina and the nodes of the ridge on 
the outer side of right forms and on the inner side of left forms. The nodes on the 
outer side of right forms, and on the inner side of left forms, become elongated to form 
ridges in later development, as also, in further development, do the anterior nodes of 
the outer side of left forms and on the inner side of right forms. Many bizarre forms 
can be formed as a result of the thickening of the ribs, but in most adults the trough 
at the anterior extremity of the outer side of left forms and on the inner side of right 
forms can still be detected. 
A count was made of specimens of Pseudopolygnathus from a horizon 65 feet above 
yYoyy FS 6 Sul. TAZ. Beree, S 
Nhhyh4 When 4% %%, 1 laleag 444% NOK 
SPECIMENS 
Number of nodes on right side of platform. 
Left side of the platform expanded. 
Left side of the platform expanded with crenulate edge. 
Left side of the platform has nodes at the posterior and a trough at the anterior 
Left side of the platformis nodose 
aS. a Ot BS AN SS 
Ridges on both sides of the platform. 
Fic. 41. Frequency diagram showing variation in the number of nodes and platform 
ridges in relation to platform shape in growth stages of a pseudopolygnathid population 
from a sample 65 ft. above the base of Z; in the Avon Gorge. 
