180 BRITISH AVONIAN CONODONT FAUNAS 
shorter than the platform. It is situated medially and bears from 4 to 8 denticles, 
the posterior-most being twice as large as the remainder. There is a general decrease 
in size anteriorly, and the denticles are more or less erect. The base of the blade is 
grooved. 
Each side of the platform bears a marginal row of laterally elongate nodes, which 
are transversely paired. These are replaced posteriorly by a single series of medial 
transverse ridges, composed of two nodes. An additional node is generally 
developed on the left side of the platform (viewed from the posterior). This addi- 
tional node, situated at the anterior end of the platform, is developed on the left 
margin of the platform, irrespective of whether the unit is laterally curved to the 
right or the left. The number of nodes in each row ranges from 4 to 11. The unit is 
arched slightly in lateral view and the platform has a nodose edge. 
In aboral view the cavity is slightly asymmetrically expanded, the inner half being 
shorter and more inflated. The base of the cavity is grooved. 
Remarks. Although P. variabilis possesses a variable number of nodes on both 
the blade and platform, calculations of denticle density of both blade and platform 
and the construction of a scatter diagram of this information, suggest that this vari- 
tion is continuous (see Fig. 35). The additional node at the anterior of the inner row 
of lateral nodes on “right” specimens appears to be constant and implies that 
paired members of P. variabilis were not symmetrical. Other examples of this 
asymmetry are to be seen in the genera Cavusgnathus, Scaphignathus, Mestognathus 
and Pseudopolygnathus. 
SCATTER DIAGRAM FOR DENTICLE DENSITY 
OF PATROGNATHUS VARIABLIS. 
2o ° 
5m 
Cc 
o 
vD o ° 
w + 
9g ° 8 t 0 
Ee o &+tot 
o + Boe + 
v0 0 OF els 
v @ eo 
UD +o 9%o8 
2 + 0@ 
D *-*KL2 22PLOTS 
10 
° O*KL3 S4PLOTS 
15 
20 30 
platform denticle density 
BLADE DENTICLE DENSITY « No OF BLADE OENTICLES 
LENGTH OF BLADE IN MM 
PLATFORM DENTICLE DENSITY =No.OF PLATFORM DENTICLES 
LENGTH OF PLATFORM IN MM 
Fic. 35. Scatter diagram to show the relationship between the density of denticles on the 
blade and those on the platform of the species Patrognathus variabilis. 
