BRITISH AVONIAN CONODONT FAUNAS 205 
the base of the Z; Limestone in the Avon Gorge. The number of denticles on the 
right side.of the platform was plotted, and also the character of the left side of the 
platform as marked by aa ridge,b a crenulated ridge,c a few denticles at the posterior 
and a trough at the anterior, or d nodes along its entire length. Adults were identi- 
fied by the presence of ridges on both sides of the platform. The results showed that 
a. Forms with 4 nodes on the right side of the platform had either a ridge, or a 
crenulate ridge on the left side of the platform. No specimens were seen which 
had 4 nodes on the right side and nodes on the left side. 
b. Forms with 5 nodes on the right side of the platform had either a crenulate ridge, 
or a few denticles at the posterior end and a trough at the anterior end. No 
specimens were found with a straight edge to the ridge, or with denticles 
continuous on the left side. 
c. Forms with 6 denticles on the right side had either a few denticles at the posterior 
extremity of the left side and a trough at the anterior extremity, or they had a 
continuous row of denticles on the left side. 
A similar plot was made of specimens higher in the section from Sample Z 21 at the 
base of Ze (fig. 25b). In this sample fewer specimens with 4 nodes on the right side 
of the platform were present (4°%, as opposed to 25 % in the lower horizon.) 
“4% %%%% %Y%N% Ahlr? ee 
i 2) 73% (@G% 423% 12473% Be <cBECIMENS 
(o) 
2 
4 
6 
8 
10 
1 * : 
Ye Number of nodes on theright side of the platform. 
Al Left side of the platform expanded. 
7) Left side of the platform expanded with a crenulate edge. 
4 Left side of the platform has nodes at the posterior anda troughat the anterior 
4 Left side of the platform is nodose. 
% Ridges on both sides of the platform. 
Fic. 42. 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 Sample Z 21 at the base of the Zz Subzone of the Avon Gorge. 
