96 BRITISH AVONIAN CONODONT FAUNAS 
non 1958 Gnathodus commutatus commutatus (Branson & Mehl) Lys & Serre: 8o1, Pl. 9, 
figs. 2a, b (=G. symmutatus). 
1959 Gnathodus commutatus commutatus (Branson & Mehl) Voges: 281. 
1960 Spathognathodus commutatus Branson & Mehl ; Clarke : 109, Pl. 3, figs. 4, 5. 
1960 Gnathodus commutatus commutatus (Branson & Mehl) Serre & Lys : 39, fig. 3. 
1961 Gnathodus commutatus (Branson & Mehl) Rexroad & Burton : 1153, Pl. 1309, figs. 
I-3. 
1961 Gnathodus commutatus var. commutatus (Branson & Mehl) Higgins: 212, Pl. 10, 
figs. 6, text-fig. 1a, (left figure only). 
1962 Gnathodus commutatus commutatus (Branson & Mehl) Higgins: PI. 2, fig. 22. 
1962 Gnathodus commutatus commutatus (Branson & Mehl) Meischner : 31, text-fig. ro. 
1963 Gnathodus commutatus commutatus (Branson & Mehl) Bouckaert & Higgins: 17, 
fig. 3. 
1964 Gnathodus commutatus pellaensis Rexroad & Furnish : 671, Pl. 111, fig. 3. 
Gnathodus commutatus (Branson & Mehl) Collinson & Druce in press. 
MATERIAL. 425 specimens : figured, X 418, X 95, X 96, X 97. 
RANGE. North Crop CYD 7-3D 22, Avon Gorge S 53. 
DESCRIPTION. This species has recently been redefined by Collinson & Druce 
(in press) and it is their revised description which is accepted here. Although the 
species, which is represented by many specimens in the present collections, shows 
appreciable variation, the distinctive characteristics are the regular sub-rectangular 
outline of the blade when seen in lateral view, the regular height and thickness of the 
confluent denticles which make up the blade, the rather square anterior profile of the 
blade, and the small posteriorly restricted sub-oval to sub-circular unornamented 
platform. This platform is always asymmetrical in detail, and shows considerable 
variation in its basal outline and in the degree of flexure of its basal margin. This 
variation is so obviously transitional, however, that it seems impossible to dis- 
tinguish any discrete categories on the basis of it. The chief variation is in the 
general form of the posterior platform, which varies from slenderly sub-elliptical to 
sub-circular, in the degree of symmetry of the posterior platform, which is generally 
marked by the inner margin being wider anteriorly than it is posteriorly, and in the 
relation of the carina to the posterior margin of the platform. In some cases the 
posterior margin of the platform tends to be relatively elongated, while in others it 
tends to be bluntly rounded. The carina may, in some specimens, extend slightly 
beyond the posterior margin when seen in oral view (PI. 19, fig. 9d) but in others it 
terminates anteriorly to the margin. In a few cases the posterior portion of the 
carina is strongly deflected laterally. In most individuals the central blade tends to 
become thicker posteriorly, where it forms the carina of the posterior platform (e.g. 
Pl. 19, fig. rod). The surface of the posterior platform is smooth and the platform 
itself occupies only the posterior quarter or third of the total length of the unit. 
The broad variation within this species noted by Collinson and Druce is also shown 
by the present specimens. 
REMARKS. In many specimens the blade is straight, but in others it is gently 
curved in a horizontal plane ; in these cases the wider of the two platforms in the 
anterior position is always that on the concave side of the blade. 
