230 BRITISH AVONIAN CONODONT FAUNAS 
ing of about 16 fused, laterally compressed denticles, those in the posterior third of 
the unit tending to be the largest. The oral profile, posterior to the blade, is gently 
convex. Lateral denticles are developed on the outer side above the basal cavity 
and number from 2 to 5 ; they tend to form a short platform, restricted to the region 
of the basal cavity, rather than occurring as laterally divergent peg-like denticles. 
The aboral lateral profile is conspicuously straight, being arched behind the cavity. 
In aboral view the cavity is ovate, slightly asymmetrical, the flared side being the 
narrower. The long axis is parallel to the long axis of the unit, equal to, or only 
slightly greater than, the maximum lateral width of the cavity. The cavity may 
extend as a faint groove for a short distance along the anterior and posterior blades. 
REMARKS. Ziegler (1960A, Pl. 3, figs. 8, 9) described forms referred by him to S. 
tridentatus, which appear to be referable to S. plumulus plumulus. The form figured 
by him (1960A, Pl. 3, fig. 7) as S. sulciferus is distinct from Branson & Mehl’s type 
specimen which is, in fact, a senior synonym of S. spinulicostatus (E. R. Branson 
1934). Ziegler’s specimen appears from its tall blade and the nature of the anterior 
face to be closely related to S. plumulus plumulus, but its lack of platform develop- 
ment precludes its inclusion in that species, although it may be a form ancestral to 
spathognathodids with high plume-like blades. 
S. plumulus plumulus is probably ancestral to Clydagnathus (see p. 85). The 
lateral profile of the blade, with its sharply descending oral surface, short anterior 
edge, and rounded antero-aboral angle, is also very similar to that of Scaphignathus. 
Spathognathodus plumulus nodosus sp. et subsp. nov. 
Plate 1, figs. 3a—4c 
DERIVATION OF NAME. From the additional nodes on the inner side of cup. 
Diacnosis. Subspecies of S. plumulus with one or more nodes developed on inner 
platform expansion. 
MATERIAL. 14 specimens: Holotype X 382, Paratype X 383. 
TYPE LOCALITY AND HORIZON. R. Clydach, Nr. Gilwern, Lower K Zone North 
Crop. Sample KL 2. 
RANGE. North Crop KL 1-KL 6, Avon Gorge K-8. 
DeEscriPTION. This form is similar to S. plumulus plumulus in gross morpho- 
logical detail, but the inner oral lip of the expanded cavity bears one or more nodes. 
Remarks. As in the S. tvidentatus group, the S. plumulus group develops lateral 
denticles on the inner and outer lateral margins of the cup. In S. plumulus nodosus 
the development towards a pseudopolygnathid morphology (as in the S. aculeatus 
and S. tvidentatus groups) is seen. The development of the denticle ranges from 
weak to strong. 
Spathognathodus plumulus shirleyae subsp. nov. 
Plate 1, figs. 7a—8c 
DERIVATION OF NAME. After Mrs. Shirley Osborn. 
