224 BRITISH AVONIAN CONODONT FAUNAS 
edge, the antero-aboral angle being bluntly rounded to angular and conspicuous. 
The median denticles, posterior to the blade, are confluent for their whole length, 
their individual tips being only rarely discernible. Those in the median two-thirds 
of the unit are of uniform height, the posterior ones being shorter, and having 4 to 6 
conspicuous apices. 
The basal cavity is longitudinally restricted but makes a conspicuous feature in 
lateral view. 
In oral view, the blade is seen to be gently curved in a horizontal plane, although 
only left forms are known. The denticles are conspicuously laterally expanded to 
give a platform-like appearance to the whole unit. The size of the platform increases 
during ontogeny (cf. Pl. 4, figs. 1a & 4a). There are up to 14 transverse denticles 
developed, those near mid-length being the widest, giving the platform a general 
biconvex outline in oral view. Deep transverse troughs separate the denticles, and 
there is a tendency for a feeble longitudinal trough to develop also. 
In aboral view the asymmetrical laterally expanded basal cavity is a conspicuous 
feature, both its lateral expansion and its asymmetry increasing in ontogeny. Its 
transverse axis lies obliquely to the longitudinal axis of the unit, the outer (convex) 
extension pointing more posteriorly. It extends to the posterior end of the unit as a 
deep and wide but gradually narrowing cavity, and as a slit-like groove along the 
anterior part of the blade. 
Spathognathodus coaptus (Branson & Mehl) 
Plate 7, figs. ga—1Ic 
1934 Spathodus coaptus Branson & Mehl: 275, Pl. 22, fig. 16. 
1938 Spathodus elongatus Branson & Mehl (partim) : 139, Pl. 23, fig. 6 only. 
1939 Spathodus crassidentatus Branson & Mehl ; Cooper: 413, Pl. 45, fig. 19. 
1939 Spathodus aciedentatus E. R. Branson ; Cooper : 413, Pl. 45, figs. 26, 28, 44. 
1939 Spathodus chouteauensis Cooper : 413, Pl. 45, fig. 20. 
1939 Spathodus isus Cooper : 413, Pl. 45, fig. 33. 
1939 Spathodus strigilis Huddle ; Cooper: 416, Pl. 45, fig. 37. 
1939 Spathodus sulciferus Branson & Mehl ; Cooper: 416, Pl. 45, figs. 17, 18. 
1949 Spathognathodus sulciferus (Branson & Mehl) Youngquist & Patterson : 72, Pl. 15, fig. 1. 
1949 Spathognathodus crassidentatus (Branson & Mehl) Youngquist & Patterson : 71, Pl. 15, 
fig. 2. 
1949 Spathognathodus aciedentatus (E. R. Branson) Youngquist & Patterson: 17, Pl. 15, 
fig. 3. 
? 1949 Spathognathodus aciedentatus (E. R. Branson) Thomas : 412, Pl. 4, fig. 7. 
1951 Spathognathodus macey (Branson & Mehl) Youngquist & Downs: 791, Pl. 3, figs. 
iis we 
1958 Spathognathodus crassidentatus Branson & Mehl ; Freyer : 85, Pl. 6, fig. 138. 
1964 Spathognathodus crassidentatus Branson & Mehl; Rexroad & Scott: 48, Pl. 3, figs. 
TS 
MATERIAL. 68 specimens : figured, X 453, X 454, X 436. 
RANGE. Avon Gorge Z 1-Z 38. 
DESCRIPTION. This is a straight bladed spathognathodid, with a straight aboral 
