BRITISH AVONIAN CONODONT FAUNAS 239 
laterally expanded denticles developed on the outer (convex) side of the main blade. 
These are barely visible in lateral view, but are conspicuously elongated blunt 
denticles in oral view. On the inner (concave) side posterior to the basal cavity, there 
is a series of up to 7 broadly similar denticles developed. The two sets of denticles 
tend to form a more or less continuous ridge across the posterior platform which they 
produce, although in some specimens they are arranged at such an angle to one 
another to give an arrow-like oral view. The basal cavity is conspicuously laterally 
expanded in oral view, the inner end of its long axis, which lies oblique to the main 
axis of the unit, lying anterior to that of the outer side. 
In aboral view the cavity is laterally expanded, the greatest width lying near its 
anterior end. It is shallow and asymmetrical and tapers more or less uniformly 
towards the posterior termination of the unit, having an overall lanceolate lachry- 
form outline, the anterior end being rounded. 
Remarks. This species differs from S. bischoffi in the development of an accessory 
row of posterior lateral denticles. Like that species, no mirror images of forms are 
known. In this the two species resemble many species of the genus Pseudopoly- 
gnathus, to which both are closely related in their overall morphology. 
Spathognathodus sp. A 
Plate 4, figs. 12a—c 
MATERIAL. 4 specimens : figured, X 405. 
RANGE. North Crop KL 3-KL 5. 
DeEscrIPTION. The unit is arched, relatively short and deep. The major denticles 
or pair of denticles occur in the anterior quarter, being massive, tall, and laterally 
compressed. The anterior blade occurs anterior to this denticle and is formed of 
denticles developed in a fan-like manner on both the oral and anterior faces of the 
unit. The median part of the blade is fairly level, but the posterior part decreases in 
Fic. 47. The development of Pseudopolygnathus postinodosus from a spathognathodid 
ancestor by addition of lateral denticles. 
