BRITISH AVONIAN CONODONT FAUNAS 83 
asymmetrical in detail, the inner side being rather wider than the outer, and some- 
times starting a little anteriorly to the outer. The widest part of the cavity occurs 
in the anterior third. Itis extended posteriorly as a narrow, elongate pointed cavity. 
It is deepest in its anterior half. 
In oral view the unit tapers regularly towards the posterior end, the narrowing 
being especially conspicuous in the posterior fifth. It is widest near the position of 
the largest denticle on the anterior blade and its sides are more or less straight, 
though in a few specimens they may be gently convex. The inner and outer 
parapets are ornamented by a number of transverse, parallel, straight ridges, which 
tend to be node-like in younger forms. They are most strongly developed in the 
anterior half and are relatively short, not reaching the broad, shallow, concave 
trough, which extends along the median part of the platform. There is a tendency 
for a slight central carina with two or three nodes to develop in the posterior one- 
fifth or one-sixth of the unit. There is a very conspicuous median depression on the 
inner side of the anterior blade and the inner anterior margin of the platform is 
strongly deflected inwards. In lateral view the oral and aboral edges are gently 
convex and the outer parapet is very slightly higher than the inner. 
The anterior blade slopes sharply downward from the largest denticle on the 
posterior end. Its anterior aboral margin is bluntly spatulate and it bears a series of 
inconspicuous node-like serrations along its length. These tend to gradually 
increase in size towards the posterior end, the posterior denticle being enormously 
expanded. It is not very high, but it is greatly elongated anterio-posteriorly, so 
that it resembles a shark fin, with a convex anterior edge and a straight or concave 
posterior edge. It is bluntly pointed, with more or less flat lateral faces and blunt 
anterior and posterior edges. The overall profile of the anterior blade may be gently 
convex or straight. It is about one-third the total length of the unit, but only about 
half of it is actually free of the platform. The anterior and posterior edges of the 
unit are almost vertical. 
REMARKS. Rexroad (1958) remarked on the changes which occurred during 
ontogeny. Similar ontogenetic changes have been noted in the present study. The 
young specimens tend to be narrow, with parapets which are nearly straight. In 
more mature specimens the platform broadens without a corresponding increase in 
length and the flare of the inner lip of the basal cavity also increases. The number of 
small denticles on the anterior blade, anterior to the prominent posterior denticle, 
also increases. A feature of the juveniles, which Rexroad did not note, was their 
tendency to develop a row of nodes on the parapet, rather than ridges. 
Cavusgnathus unicornis resembles C. charactus and C. convexus in the convex out- 
line of the platform in lateral view. However C. wnicornis is distinctive because of 
the high posterior denticle of the anterior blade. 
There is a tendency in some specimens for the outer lateral face to be offset near 
the position of the largest denticle. The carina in the posterior portion of the 
platform seems to occur only in the largest individuals. 
Young individuals of this species from the present faunas agree with Rexroad’s 
description (1958 : 17). Older individuals in our D collections have been identified 
