BRITISH AVONIAN CONODONT FAUNAS 95 
1960 Gnathodus smithi Clarke : 26, Pl. 4, figs. 13-14 ; Pl. 5, figs. 9 & Io. 
1961 Gnathodus bilineatus (Roundy) Higgins: PI. to, fig. 5. 
1962 Gnathodus bilineatus (Roundy) Higgins : (partim), Pl. 2, fig. 25 only. 
1962 Gnathodus bilineatus bilineatus (Roundy) Meischner : 31, fig. ro. 
1965 Gnathodus bilineatus (Roundy) Dunn: 1148, Pl. 140, figs. 7-9. 
Gnathodus bilineatus (Roundy) Collinson & Druce in press. 
MATERIAL. 765 specimens, North Crop : figured X 416, X 417, X 93, X 94. 
Rance. North Crop 3D 10-3D 23. 
DEscRIPTION. This is one of the most abundant species in the higher parts of the 
section of the North Crop and, although it displays considerable variation, its main 
features agree closely with those described by earlier authors, especially Hass (1953) 
and Bischoff (1957). There is considerable variation in the ontogeny of this 
species. The main variation concerns the increase in the relative size and angularity 
of outline of the outer lateral platform, and its increasingly strongly developed 
ornamentation. In young forms this tends to be inconspicuously nodose and 
rather irregular, but in older forms it becomes either longitudinally linear or 
broadly concentric in disposition, and the nodes become stronger, tending to form 
elongate ridges. There is also considerable variation in the strength of the inner 
lateral platform and of the transverse denticles developed upon it. In numbers of 
specimens it is pinched inwards in the anterior half when seen in oral view. There is 
also appreciable variation in the depth and relative width of the sulcus that separates 
these denticles from the carina. In spite of its extent, the variation appears to be 
continuous, and there is no obvious difference between specimens collected from 
different parts of the range of the species. 
Gnathodus commutatus (Branson & Mehl) 
Plate 19, figs. ga—-12d 
1941 Spathognathodus commutatus Branson & Mehl : 98, Pl. 19, figs. 1-4. 
1941 Spathognathodus commutatus Branson & Mehl; Ellison & Graves: 3, 4. Pl. 2, 
fig. 6, (non Pl. 2, fig. 4=Gnathodus symmutatus?). 
1953  Gnathodus inornatus Hass : 8o, Pl. 14, figs. 9-11. 
1956 Spathognathodus commutatus Branson & Mehl; Elias: 1109, Pl. 3, figs. 19-22. 
1956 Spathognathodus inornatus Hass ; Elias: 1109, Pl. 3, figs. 37-39. 
1956 Spathognathodus cf. inornatus Elias : 1109, Pl. 3, figs. 41, 42, (non Pl. 3, figs. 62, 63=G. 
symmutatus?). 
1957 Gnathodus commutatus commutatus (Branson & Mehl) Bischoff : 22, Pl. 4, figs. 2-6, 
15. 
1957 Spathognathodus cf. S. commutatus Branson & Mehl ; Rexroad : 38, Pl. 3, figs. 23, 24. 
1957 Gnathodus commutatus commutatus (Branson & Mehl) Ziegler in Flugel & Ziegler 
39, Pil 3; fig. 21. 
1958 Gnathodus inornatus Hass ; Stanley : 465, Pl. 68, figs. 5, 6. 
1958 Spathognathodus cf. S. commutatus Branson & Mehl ; Rexroad : 26, Pl. 6, fig. 8. 
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