BRITISH AVONIAN CONODONT FAUNAS 85 
Many adult specimens of Scaphignathus (illustrated by Helms 1959) have a similar 
appearance to juveniles of Clydagnathus. The present genus can be distinguished 
by the lack of a carina on the oral surface of the platform (except at the posterior 
extremity) and the distinct basal cavity. Clydagnathus can be distinguished from 
Cavusgnathus by the general anterior closure of the oral trough, by the merging of the 
marginal ornament with the blade, and by the lateral, rather than longitudinal, 
expansion of the cavity. It is thought that Clydagnathus was derived from Spatho- 
gnathodus plumulus plumulus by addition of nodes and lateral movement of the 
blade. Scaphignathus probably arose from a polygnathid ancestor and Cavusg- 
nathus was derived from Taphrognathus (Rexroad and Collinson 1963). Thus none 
of the three broadly homoeomorphic genera Scaphignathus, Clydagnathus and Cavusg- 
nathus is related genetically and their different positions in the stratigraphical 
column justify the use of distinct generic nomenclature. The use of distinct generic 
names is a reflection of their distinct phylogenetic origin as well as an aid to strati- 
graphy, although the degree of morphological difference between them is less than 
that between most other platform genera. 
Clydagnathus cavusformis sp. nov. 
Plate 1, figs. gQ-13d 
DERIVATION OF NAME. From the close resemblance to the genus Cavusgnathus. 
Diacnosis. Clydagnathid with lateral anterior blade very short, plume-like and 
sub-triangular in lateral profile, consisting of about four to six fused denticles with 
free, bluntly chevron shaped tips, largest near the posterior end. 
- Platform 
Anterior Posterior 
Anterior aboral tip 
Inner lateral view 
Outer side 
> Carina 
Medial trough 
Basal cavity 
Oral view Aboral view 
Fic. 20. Clydagnathus sp. showing morphological terms used in the text. 
