BRITISH AVONIAN CONODONT FAUNAS I5I 
posterior. This tends to be sub-triangular in form, and is inclined posteriorly, with 
strongly convex lateral faces and blunt anterior and posterior edges. Its inner 
lateral face is flatter than its outer. The other denticles stand more or less erect to 
the bar but their inclination is rather variable. The anterior blade is inclined gently 
inwards ; there is a tendency for the denticles, to show some reduction in height 
anteriorly and the anterior blade projects beyond the platform at the anterior end. 
In oral view a deep trough is developed parallel to the anterior blade, being widest 
and deepest anteriorly. It shallows towards the position of maximum width of the 
posterior platform and becomes obsolescent in the posterior half. It is broadly U- 
shaped. The outer margins of the carina are decorated with feeble to moderate 
transverse ornament, which tends to be aligned acutely to the length of the unit, 
forming an arrow-type structure pointing forward, rather than being developed at 
right angles. There are about 12 of these ridges on the outer side of the platform 
and about 17 on the inner. They become rapidly obsolescent towards the middle of 
the platform and are more feebly developed on the inner than on the outer side. 
There is a variable posterior carina developed in the posterior part of the unit, 
consisting of a low ridge of only barely distinguishable nodose denticles, but it may be 
extended anteriorly to meet the antero-lateral inner edge of the platform as a low 
inconspicuous ridge, dividing the platform into two unequal parts. It runs to the 
left of the medial trough. The outline of the platform in oral view is such that the 
anterior two-thirds of the inner lateral margin is straight and the posterior third 
inclined towards its pointed posterior termination. The outer lateral face is straight 
in its anterior half and gently concave in its posterior half, the whole appearance of 
the unit being pinched towards the posterior fifth. The anterior inner lateral edge 
terminates in an undenticulate projection. The edges of the platform are feebly up- 
turned. In inner lateral view the antero-inner face of the platform is flat, with sharp 
oral and aboral lateral edges. The lateral face decreases in width towards the mid- 
point, beyond which the platform has a thin lateral face which merges with the base. 
The outer lateral face is characterized by having a flat surface, parallel to the 
denticles, with a longitudinal ridge developed about the base of them, and then a 
sloping lateral face which slopes towards the basal cavity below the ridge. In outer 
lateral view the posterior half has a lateral face, which slopes steadily towards the 
basal cavity. 
Anterior blade Outer side of platform 
Platform 
Anterior Carine Posterior 
r ral face 
Inner lateral fa Trough 
Fic. 30. Mestognathus sp. showing morphological terms used in the text. 
