BRITISH AVONIAN CONODONT FAUNAS 235 
REMARKS. This species bears a marked resemblance to S. crassidentatus and was 
included in that species by Rexroad & Scott (1964), but the very different dentition 
serves to distinguish it. The medial and posterior oral parts commonly bear 30 
denticles, as opposed to the 15 or so of S. crassidentatus. Between the anterior part 
of the cavity and the anterior high blade, this species has 7 to 8 denticles whereas S. 
crassidentatus has only 2 to 3. Cooper’s holotype has a total of only 20 denticles, but 
is otherwise very similar to the present specimens. The present specimens also 
resemble those described by Cooper (1939, Pl. 45, fig. 17 only) as Spathodus sulciferus 
Branson & Mehl (fig. 18 of Cooper’s Pl. 45 is probably S. crassidentatus) and those 
described as S. macer by Youngquist & Downs (1951, Pl. 111, figs. 1, 2). S. chou- 
teauensis Cooper (1939, Pl. 45, fig. 20) is also broadly similar and also has resem- 
blances to S. crassidentatus. 
Spathognathodus cf. robustus (Branson & Mehl) 
Plate 7, figs. 6a—7c 
1934 Spathodus robustus Branson & Mehl: 180, Pl. 17, fig. 21. 
MATERIAL. 17 specimens : figured, X 387, X 388. 
Rance. North Crop KL 19-ZLA 14, Avon Gorge Kg I. 
DEscRIPTION. The unit is bar-like, being highest at the anterior end and sloping 
towards the posterior end, the oral outline being nearly straight. The anterior 
denticles tend to be massive. Smaller denticles appear on the anterior edge of the 
anteriormost massive denticle, giving a posteriorly inclined anterior edge which 
makes a sharp angle of about 60°, with the aboral edge. The denticles in the median 
part of the unit are fused, two denticles often fusing into one free tip. The posterior 
bar-like part of the unit is shallow, twisted and depressed downwards, the denticles 
tending to be isolated and free standing and curved towards the inner side. 
In aboral view the cavity is fairly large, elongate, medially situated, about twice as 
long as wide, the wider anterior end narrowing rapidly towards the anterior of the 
unit and the other end narrowing slowly towards the posterior ; the lips are 
thickened slightly. 
Remarks. Branson & Mehl (1934, Pl. 17, fig. 21) described a form very similar to 
ours as S. vobustus. The dentition is very similar, except that in their specimen the 
massive anterior denticle and the isolated posterior denticles are less conspicuously 
developed than those of the present specimens. 
There appears to be a morphological trend from these specimens through such 
forms as Spathognathodus sp. B. to a pseudopolygnathid form. 
The Spathognathodus tridentatus group 
Much confusion has arisen in recent conodont literature (Sannemann 1955, 
Bischoff & Ziegler 1956, Freyer 1961 and Ziegler 1962) with regard to specimens of 
the genus Spathognathodus which develop lateral denticles. Spathognathodids with 
