BRITISH AVONIAN CONODONT FAUNAS 199 
DESCRIPTION. The most striking features of this species are the confluent denticles 
of the long anterior bar, and the large denticles of the posterior bar. There is no 
conspicuous apical denticle, and the denticles of the posterior bar are discrete. 
In aboral view the unit is straight in its median two-thirds, but both the anterior 
and the posterior ends are sharply flexed inwards. There is a deep and relatively 
conspicuous basal cavity below the apical denticle, and this is extended as a wide 
deep groove along the greater length of the anterior bar, and as a shorter narrowing 
groove below about half the length of the posterior bar. The edges of the cavity and 
of the aboral surfaces of the bars are relatively thin. 
This is a somewhat variable species, but the anterior bar is generally rather longer 
and deeper than the posterior, and tends to be rather straight along its aboral margin 
when seen in inner lateral view. It bears a series of 9 or 10 laterally compressed 
denticles which are confluent at their bases and which are inclined posteriorly. In 
some specimens they show an alteration in size, and they also show a general tendency 
to increase in size posteriorly. Their free edges are sharp and their lateral faces are 
strongly convex, especially in the posterior half of the anterior bar. They show 
some variation, in that in some specimens they are free for the greater part of their 
length, but this is a relatively unusual feature. 
There is no apical denticle in the strict sense, but the 2 or 3 denticles at the point 
of flexure of the unit tend to be larger than any of the other, and to be more or less 
equalinsize. They are free for most of their length and they are inclined posteriorly, 
the degree of inclination increasing towards the posterior end of the series. They 
have sharp anterior and posterior edges and all taper sharply to their pointed tips. 
They have gently convex lateral faces. Behind them is a series of up to 5 discrete 
and strongly posteriorly inclined denticles, which in some specimens have smaller 
denticles separating them. There is a rather indistinct apical lip at the point of 
flexure. The posterior bar tends to decrease in depth posteriorly and its posterior 
aboral margin is gently curved. 
Prioniodina ? sp. nov. 
Plate 28, figs. 6a—c 
MATERIAL. I specimen : figured, X 345. 
Rance. Scotland GILM 3. 
DescripTion. A single specimen is tentatively referred to the genus Prioniodina. 
It is characterized by an elongate straight anterior bar, which is continuous, without 
any vertical flexure with the straight, but broken, posterior bar. The anterior bar 
has convex lateral shoulders on the inner lateral face, and a straight inner lateral face 
below them. Its oral surface bears 5 discrete short denticles with sharply pointed 
tips, and posterior edges which are sharp ; they taper uniformly from their point of 
origin and stand more or less erect or only slightly inclined to the anterior bar. 
They show a slight tendency to increase in height posteriorly. The posterior bar 
bears at least 3 closely spaced, but discrete, sharply-pointed denticles. That 
nearest to the apical denticle is larger than the other two. They are strongly 
