BRITISH AVONIAN CONODONT FAUNAS 237 
Spathognathodus tridentatus (E. R. Branson) 
Plate 3, figs. ga—12c 
1934 Spathodus sulciferus Branson & Mehl; E. R. Branson: 304, Pl. 27, fig. 15 ? 22 
(non Pl. 27, fig. 20 = S. costatus sulciferus). 
1934 Spathodus tridentatus E. R. Branson : 307, Pl. 27, fig. 25. 
1934 Spathodus duplidens Huddle : 91, Pl. 12, figs. 1-4. 
1949 Spathognathodus costatus (E. R. Branson) Thomas : 409, 412, Pl. 4, fig. ro. 
1949 Spathognathodus tridentatus (E. R. Branson) Thomas : 412, Pl. 4, fig. 11. 
non 1955 Spathognathodus tridentatus (E. R. Branson) Sannemann : Pl. 24, fig. 13. 
non 1956 Spathognathodus tridentatus (E. R. Branson) Bischoff & Ziegler : p. 167, Pl. 13, 
LISS.) Tie 
1956 Spathognathodus aciedentatus (E. R. Branson) Hass PI. 2, fig. 26. 
1957 Spathognathodus aciedentatus (E. R. Branson) Cloud, Barnes & Hass: PI. 5, 
fig. 7. 
21959 Spathognathodus aciedentatus (E. R. Branson) Hass: Pl. 49, fig. 24. 
non 1959 Spathognathodus tridentatus (E. R. Branson) Voges : 658, Pl. 3, fig. 7. 
non 1960 Spathognathodus tridentatus (E. R. Branson) Dvorak & Freyer: PI. 2, figs. 9, 10. 
non 1961 Spathognathodus tridentatus (E. R. Branson) Freyer : 89, Pl. 6, fig. 150. 
non 1961 Spathognathodus tridentatus (E. R. Branson) Remack-Petitot : 261, fig. 4. 
mos a, 
MATERIAL. 259 specimens : figured, X 394, X 395, X 396, X 397. 
RANGE. North Crop KL 19-ZLA 27, Avon Gorge K 21-Z 24. 
DEscRIPTION. The unit is straight to gently arched in the posterior part and 
slightly bowed ; it is highest at the anterior. The anterior blade is composed of 3 to 
4 high denticles, the highest being the anterior-most 2 or 3 denticles. The median 
denticles are of equal height, and the unit shallows posteriorly, the denticles becoming 
shorter, less fused, and rather sharper towards the posterior end. 
On the inner side there are developed from 2 to 4 lateral denticles, situated above 
the cavity. They are peg-like and are not connected to the main blade by transverse 
ridges. 
In aboral view the basal cavity is large and symmetrically expanded, the widest 
part being at or just anterior to the mid-point. The cavity extends in either direc- 
tion as a short aboral groove, the posterior groove being longer than the anterior. 
REMARKS. This is a very common species with a limited range in our faunas. It 
is closely related to and occupies an intermediate phylogenetic position between S. 
anteposicornis and S. costatus costatus. It is also very close to S. aculeatus, but its 
higher stratigraphical position implies a distinct phylogenetic origin. For this 
reason it seems undesirable to regard it as a junior synonym of that species. The 
similarity between the two forms is, in any case, not exact. S. aculeatus has a more 
conspicuously laterally expanded basal cavity. Individuals from stratigraphically 
older samples (e.g. Pl. 3, figs. rra—c) tend to have a smaller number of blade denticles 
(9 to 13) behind the anterior blade than those from younger samples (e.g. Pl. 3, 
figs. 10a—c), which have from 14 to 19 denticles. 
