108 BRITISH AVONIAN CONODONT FAUNAS 
In a few specimens the highest denticles are developed above the anterior end of 
the basal cavity (e.g. Pl. 18, fig. 2c ; X 89). In all other respects, however, these 
specimens resemble the holotype. 
Gnathodus symmutatus sp. nov. 
Plate 19, figs. Ia—4c 
? 1941 Spathognathodus commutatus Branson & Mehl; Ellison & Graves: PI. 2, fig. 4 (non 
Pl. 2, fig. 6). 
? 1956 Spathognathodus cf. inornatus Hass ; Elias: Pl. 3, figs. 62, 65 (non PI. 3, figs. 41, 42). 
1958 Gnathodus commutatus commutatus Branson & Mehl; Lys & Serre: 8g1, Pl. 9, figs. 
2a, b. 
MATERIAL. 42 specimens : Holotype X 134, Paratypes X 135-7. 
TYPE LOCALITY AND HORIZON. North Crop Sample 3D 14/15. 
RANGE. North Crop CYD 7-3D 22. 
Di1aGnosis. Small gnathodids with elongate blades, and relatively little expanded 
posterior platforms, convex in outline, tapering at both ends, and unornamented. 
DescriPTIon. The blade is more or less regular in height or has a gently convex 
oral profile in the anterior portion, but for most of the length of the platform the 
denticle height decreases uniformly towards the posterior end of the unit. In 
mature specimens the anterior blade occupies about half of the total length of the 
unit ; it is often deepest at its posterior end, and the oral surface bears about g or 10 
denticles anterior to its junction with the posterior platform. These denticles tend 
to be more or less erect, discrete from about their mid points upwards, and sharply 
pointed ; they are strongly laterally compressed and are of sub-equal size, the 
largest tending to occur in the posterior portion of the anterior blade. The lateral 
faces of the bar are gently convex and the anterior aboral margin is rounded to 
bluntly angular ; the anterior edge is straight, and the bar is straight or gently 
deflected in a horizontal plane. The blade is continued posteriorly as a central 
carina on the platform, the denticles being short, and partly fused ; the total number 
of denticles in the blade and the carina is about 22. The posterior platform occupies 
the posterior half to two-thirds of the unit ; it is relatively unexpanded laterally 
and tapers uniformly at its anterior and posterior ends. It tends to be steep sided, 
the lateral faces forming an acute angle below the carina ; one face is often more 
strongly laterally expanded than the other. The surface is smooth and no ornamen- 
tation is developed, although in some specimens there is a tendency for the appear- 
ance of incipient marginal denticulation, similar to that in G. homopunctatus. 
The basal cavity is wide and flaring, and is continued anteriorly as a narrow groove 
on the base of the anterior blade. The growth of the posterior platform in mature 
specimens tends to reduce the angle which its lateral faces form at their junction. In 
most specimens its depth is about equal to the height of the carina on its surface. 
