78 BRITISH AVONIAN CONODONT FAUNAS 
For purposes of description, the blade is directed anteriorly. It is continued 
posteriorly as the outer edge of platform, the blade parapet. The elevated inner edge 
of the platform is the inner parapet.’ 
REMARKS. Cavusgnathus bears a marked resemblance to the genus Mestognathus, 
but the two are easily distinguished on the basis of their basal cavities, that of 
Mestognathus being small and narrow and that of Cavusgnathus wide and flaring. 
The anterior blade in the cavusgnathid group has hitherto been thought to be 
confined to the outer side of the unit and to be lateral in position (Rexroad in 
Lindstré6m 1964 : 124). However, the present study has shown that the position 
of the blade in Cavusgnathus varies through the section. In the K Zone it is both 
medial and lateral in position, whereas in the Z to D Zones it is lateral in position. 
Some forms in the Z Zone have been found with the blade on the inner side. 
In North America it has hitherto been thought that Cavusgnathus developed from 
Taphrognathus in the Late Valmeyeran (Rexroad and Collinson 1963). Cavus- 
gnathus, however, makes its first appearance in the Avon Gorge in the basal beds of 
Platform 
neti f *? — Posterior 
nterior = soe ane 
Aboral edge 
Inner lateral view 
Outer side 
ye Oral surface 
Basal ao wer 
covity Central nis i 
trough — 
Lateral 
ridges Carina 
Aboral view Oral view 
Fic. 19. Cavusgnathus sp. showing morphological terms used in the text. 
