206 BRITISH AVONIAN CONODONT FAUNAS 
This seems to indicate that the number of denticles on the right side of the platform 
is indicative of the ontogenetic age of the specimen and that the age is also reflected 
on the left side by the stage of development, through ridge, crenulated ridge, nodose 
posterior part of ridge and a continuously nodose ridge, culminating with the 
presence of ridges on both sides of the platform. 
The stage of development in Voges’s fauna represented by Pseudopolygnathus 
triangulus inaequalis and Pseudopolygnathus triangulus triangulus, which developed 
from the stage represented by Pseudopolygnathus foliaceus and Pseudopolygnathus 
apetodus, is not present in the Avonian. In the Avonian, specimens of Pseudopoly- 
gnathus primus are developed which consist of 7 ridges on the right side ofthe platform 
and a number of nodes confined to the outer margin on the left side of the platform, 
with a trough developed between the carina and the nodes of the left side of the 
platform. With further development, the nodes on the outer margin of the left side 
of the platform extend towards the carina to become ridges similar in outline to those 
on the right side of the platform. This development starts at first at the posterior 
end and extends towards the anterior, with the result that the platform on both sides 
of the carina consists of a number of ridges. The left side of the platform never 
extends as far to the anterior as the right side of the platform. In addition, a faint 
trace of a trough is present at the side of the carina, even in adult specimens. These 
forms are identical to Pseudopolygnathus multistriatus (Mehl and Thomas). 
Higher in the section forms here referred to Pseudopolygnathus cf. longiposticus 
appear and they have the same outline as advanced species of P. primus ; it is 
suggested, therefore, that they may represent a further stage in the development of 
Pseudopolygnathus. 
The third development of Pseudopolygnathus in the Avonian—P. multistriatus 
development—takes place in Zz. The specimen illustrated by Hass (1959, Pl. 47, 
fig. 21) as a juvenile of Pseudopolygnathus lanceolatus is a straight-bladed spatho- 
gnathodid, with three lateral denticles. From such individuals the ontogenetic 
growth stages illustrated by Hass lead to the development of P. multistriatus. 
In juveniles (which resemble Spathognathodus tridentatus) the denticles are 
developed on one side of the platform only ; the opposite side develops at first by the 
formation of a slight ridge and, later, by the development of nodes on the ridge. A 
slight trough is again present on one side and this side is not developed as far to the 
anterior as is the other. Later development involves the replacement of nodes by 
ridges, which in adults become coarse and irregular. 
Difficulty was experienced in the present study in separating juvenile specimens of 
P. multistriatus from adult specimens of Pseudopolygnathus dentilineatus. Specimens 
in the ontogenetic sequence of P. multistriatus, which are similar to Pseudopoly- 
gnathus striatus Mehl and Thomas, are identical to P. dentilineatus of Ziegler (1962). 
Bischoff (1957) and Voges (1959) include P. striatus in synonomy with P. dentilineatus. 
In view of the fauna associated with P. striatus, including Gnathodus texanus and 
Gnathodus cunetformis, it seems likely that Rexroad and Scott’s (1964) interpretation 
in placing P. striatus in synonomy with P. multistriatus is correct. This difference 
