BRITISH AVONIAN CONODONT FAUNAS 79 
the Z Zone and an earlier origin must therefore be suggested. The writers believe 
that Cavusgnathus may have had its origin in Spathognathodus plumulus sp. nov. 
Cavusgnathus charactus Rexroad 
Plate 13, figs. 6a—7d, 13a—c 
1957 Cavusgnathus chavacta Rexroad : 15-16, Pl. 1, figs. 1, 2. 
1961 Cavusgnathus chavacta Rexroad ; Rexroad & Collinson : Pl. 1. 
1963 Cavusgnathus chavacta Rexroad ; Rexroad & Collinson : 8, Pl. 1, fig. 29. 
MATERIAL. 8 specimens : figured, X 59, X 61, X 62. 
RANGE. North Crop CYD 7, Avon Gorge S 5-S 44. 
DEscriIPTION. The diagnostic characters of this species are the general form of the 
anterior blade, which consists of six to eight denticles with low rounded apices of 
more or less uniform size, except for those in the anterior portion. The posterior end 
of the blade is separated by a distinct undenticulated depression from the outer 
lateral face of the posterior platform. The free anterior blade is not long in com- 
parison with the length of the platform. In outer lateral view the platform decreases 
in depth towards the posterior end. Its oral surface is bluntly crenulate and the 
whole aboral surface of the unit is regularly concave. There is a conspicuous 
thickening below the basal cavity, which is situated in the anterior portion of the 
platform and the whole outer lateral edge of the blade is bevelled. In some speci- 
mens there are only four or five denticles on the anterior blade, the most posterior 
being the largest and most massive. They all stand more or less erect to the bar. 
The oral margin of the platform is continuously convex in outer lateral view, the 
outer parapet obscuring the inner, when seen in this direction. The blade tends to 
decrease in depth posteriorly ; its anterior aboral corner is bluntly rounded. In 
inner lateral view the basal cavity makes a conspicuous flaring feature on the aboral 
surface, and the oral edge of the platform is strongly convex and irregularly crenulate. 
The platform tends to show some narrowing anteriorly and its widest point is in the 
posterior half. In the posterior half there is a median carina of variable length, 
consisting of a number of rather conspicuous but fused, strongly laterally compressed 
nodes. The carina extends beyond the posterior limit of the platform proper. The 
platform edges are decorated with feeble transverse ridges and the whole median area 
of the platform is excavated by a deep U-shaped trough, which runs parallel to the 
anterior blade and does not decrease greatly in depth when traced anteriorly. 
In aboral view the aboral cavity is widely flaring and asymmetrical, but the basal 
pit itself is restricted to the anterior quarter of the platform. It is deep but not very 
broad and is extended as a longitudinal slit to the posterior end of the unit, the 
lateral edges of the lip gradually converging towards the posterior end. It is also 
extended anteriorly for a short distance, though the inner lateral lip, when seen in 
aboral view tends to be more widely flared than the outer, and also in some, though 
not in all, specimens to extend further forward. 
The total length of the blade is about one-third of the whole length of the unit. 
The blade is free for about half its length. 
