BRITISH AVONIAN CONODONT FAUNAS 195 
posterior edge of the apical denticle there is a strongly flared basal cavity which, 
although laterally wide, is not very deep. 
The whole unit is rather small and short and the apical denticle looks altogether 
too large for the general proportions of the anterior and posterior bars. The 
anterior bar is short, strongly recurved and slender in general construction, making 
an angle of about 130° with the straight basal margin of the posterior bar in outer 
lateral view. The anterior bar bears 2 or 3 small denticles with blunt free tips, but 
otherwise basally confluent. They have a tendency to increase in size posteriorly 
and stand more or less erect, or only gently posteriorly inclined, to the anterior bar. 
The anterior bar is of more or less uniform depth throughout its length, though it 
may be a little shallower anteriorly. 
The apical denticle is strongly recurved on its anterior edge, but its posterior edge 
is straight or only feebly curved. It is sharply pointed and very broad at its base, 
being about five or six times as wide as the largest adjacent denticles. It has sharp 
anterior and posterior edges and gently convex lateral faces. The posterior bar is 
short and minutely denticulate, the denticles being so closely crowded that they are 
virtually confluent, except for their blunted tips. The bar decreases in depth 
posteriorly and is slightly longer, though shallower than, the anterior bar. 
The other conspicuous feature of the unit in outer lateral view is the widely flaring 
basal cavity, which forms a conspicuous feature below the posterior half of the apical 
denticle. It is less conspicuous in inner lateral view and the whole unit is slightly 
bowed inward, having flatter faces. 
In aboral view the basal cavity is seen to be asymmetrical, being more widely 
flared on the outer lateral face than it is on the inner. On the inner lateral face, 
although there is less curvature, the convexity continues towards the tip of the 
posterior bar. The whole posterior bar, as well as the posterior half of the apical 
denticle, is excavated by the cavity, which is deepest below the posterior edge of the 
apical denticle. It is continued anteriorly as a minute slit. 
REMARKS. Collinson and Druce have discussed the relationship of this species to 
similar species of the genus Ozarkodina. 
Prioniodina laevipostica (Rexroad & Collinson) 
Plate 28, figs. 11-12b 
1963 Ozarkodina laevipostica Rexroad & Collinson : 19, Pl. 1, figs. 1-6. 
MATERIAL. 31 specimens : figured, X 316, X 317. 
Rance. North Crop ZLA 6-ZLA 12, CYD 6-CYD 7. 
DEscRIPTION. This species closely approaches P. eiveica, but differs from it in the 
relatively stronger development of the anterior and posterior bars, the less massive 
apical denticle, and the rather less conspicuous basal cavity. Rexroad & Collinson 
(1963) noted that their species had a posterior bar which was only partly denticulate, 
but both their illustrations and the present specimens show that, in at least some 
individuals, the posterior bar is denticulate for most or all of its length. The present 
