BRITISH AVONIAN CONODONT FAUNAS 203 
develops a second stage, represented by Pseudopolygnathus dentilineatus (see Voges 
1959 : 297, fig. 5). In this second stage the outer margin of the platform of right 
forms and the inner margin of the platform of left forms are convex, and they are 
thus bilaterally asymmetrical elements. The outer part of the platform of left 
forms and the inner part of the platform of right forms consist of a few nodes, 
confined to the posterior part of the platforms. The basal cavities of right and left 
forms are bilaterally symmetrical elements. The two halves of the basal cavity are 
unlike in form, being characterized on the outside by a fold of the margin and a 
lip-like indentation of the edge. This form of the basal cavity is retained during 
further developments of the oral surface of the Pseudopolygnathus primus group. 
Specimens of P. dentilineatus from the P. dentilineatus development of Voges, are 
very similar to specimens of Pseudopolygnathus from the lower and middle parts of 
the K Zone of the Avonian. These specimens are here referred to as Pseudopoly- 
gnathus vogest sp. nov. and are typical of the Cu I stage of the Sauerland (Voges 1959). 
This is the lowest development of Pseudopolygnathus in the Avonian. 
Voges was able to see the following further development of his Sauerland pseudo- 
polygnathid faunas. The nodes and ridges of the outer part of the platform in left 
forms, and on the inner part of the platform in right forms, extend towards the 
anterior end and nearer the anterior extremity of the opposite side of the platform. 
In addition, a symmetrical element is present in the ornament of the platform, when 
the inner anterior angle of the platform becomes accentuated. P. foliaceus E. R. 
Branson, and P. apetodus Cooper are representatives of this stage. 
The stage represented by P. foliaceus and P. apetodus develops into P. triangulus 
inaequalis and P. triangulus triangulus, which in the details of surface ornamentation, 
is a symmetrical element. In P. triangulus, the conspicuous asymmetry has 
disappeared and only the thickening of the blade near the platform on the right side, 
which is higher on the blade than the left, remains. 
In the uppermost part of the K Zone of the Avonian, no specimens of Pseudopoly- 
gnathus have been found. There is a second development of Pseudopolygnathus—the 
P. primus development—near the base of the Z; Limestone. The specimens of this 
development are similar, but not identical, to specimens from the stage in Voges’s 
development, which he described as including ‘ specimens similar to Pseudopoly- 
D 
Fic. 40. Diagram to illustrate the ontogenetic development of pseudopolygnathids in 
the Z; Subzone of the Avonian. 
