200 BRITISH AVONIAN CONODONT FAUNAS 
laterally compressed, with sharp anterior and posterior edges and gently convex 
lateral faces. The apical denticle is about three times as wide as the adjacent 
denticles and two to three times as long. Its sharp anterior and posterior edges 
taper uniformly to a point and it is gently inclined to the posterior bar. Along the 
whole inner lateral length of the anterior and posterior bars there is a more or less 
conspicuously convex shoulder below the point of origin of the denticles. In outer 
lateral view the whole unit is somewhat flatter and the denticles are seen to curve 
slightly inwards. In aboral view the unit is excavated by a thin slit, which is very 
narrow anteriorly, but which increases regularly in width posteriorly, being widest 
posterior to the apical denticle, though there is no sign of lateral flare on the edges 
below the apical denticle. 
Genus PSEUDOPOLYGNATHUS Branson & Mehl 1934 
1934 Pseudopolygnathus Branson & Mehl : 297. 
1939 Macropolygnathus Cooper : 392. 
TYPE SPECIES. Pseudopolygnathus prima Branson & Mehl 1934. 
THE ORIGIN AND NATURE OF THE GENUS 
Pseudopolygnathus developed from Spathognathodus in the late Devonian and early 
Mississippian. Spathognathodus anteposicornis, S. plumulus plumulus sp. nov. and 
S. aculeatus have lateral denticles developed on one side of the blade and S. costatus 
sulciferus has lateral denticles on both sides of the blade. Branson and Mehl (1934A : 
298) have reported a series of specimens which are transitional between Spatho- 
gnathodus and Pseudopolygnathus. Voges (1959 : 2096, fig. 4, & 297, fig. 5) has also 
illustrated a similar transitional series in the west German Cu I faunas. 
Homoeomorphy among recurrent laterally nodose spathognathodids within the 
present fauna, is discussed below (p. 239). Thus, since the laterally nodose spatho- 
gnathodids are homoeomorphic, and at least three chronologically distinct evolu- 
tionary lines have been demonstrated, it follows that the genus Pseudopolygnathus is 
polyphyletic. 
This can be seen in our faunas. A Lower K Zone species is known, but there is then 
a complete absence of pseudopolygnathids until the Upper Z Zone is reached in the 
North Crop, and the Lower Z Zone in the Avon Gorge. 
The Lower K Zone forms probably arose from the platform evolution of Upper 
Devonian to VI nodose spathognathodids. The Upper Z Zone forms, however, can 
be seen to have evolved directly from forms herein called S. costatus sulciferus, which 
are stratigraphically restricted to the lower and middle parts of the Z Zone. 
HOMOEOMORPHY AND TAXONOMY 
As in the spathognathodids, the existence of homoeomorphy in the pseudopoly- 
gnathids involves some problems in nomenclature. Since, however, most species of 
the genus were described by E. R. Branson (1934) from the Hannibal Formation of 
Missouri, in association with forms such as S. costatus sulciferus, it is probable that 
