BRITISH AVONIAN CONODONT FAUNAS 229 
shallow over the whole length. The denticles are highest at the anterior end, 
decreasing in height towards the position of the anterior end of the cavity, beyond 
which they increase in size towards the posterior end of the cavity, and then decrease 
gradually towards the posterior end of the unit. The denticles are compressed, fused 
at their bases, and free at their tips; about 20 in number. Longitudinal 
“shoulders ’ are developed on the lateral faces below the bases of the denticles. The 
aboral outline is straight to slightly arched. The very shallow depth of the blade 
just anterior to the basal cavity gives a ‘ sagging’ profile to the unit. The length 
of the unit is about four times the maximum depth. 
The cavity is symmetrical, elongate and shallow, with a slight appearance of cups. 
It narrows to grooves at both ends, which extend about halfway along aboral edges 
towards anterior and posterior ends. 
REMARKS. Our specimens agree perfectly with the figured holotype of S. elongatus 
(Branson & Mehl, 1938A, Pl. 34, fig.6), although their description is scarcely adequate 
to distinguish this from other species. Within our fauna there appear to be two 
forms ; the first includes those with free standing fine denticles (Pl. 7, fig. 5a) which 
tend to be sharper, longer and of more regular length than those of the second group, 
which have laterally compressed denticles (Pl. 7, fig. 1a). The long, free-standing 
denticles of this first group are reminiscent of S. denticulatus (E. R. Branson) (1934, 
Pl. 27, fig. 17), but the differences in relative length of the units and anterior dentition 
preclude us from placing these forms within that species. Cooper (1939 : 413) refers 
the paratype (Pl. 34, fig. 9) to a new species S. chouteauensis. Rexroad & Scott 
(1964 : 48) refer both the holotype and paratype to S. crassidentatus (Branson & 
Mehl). It is possible that the paratype, referred to S. chouteauensis by Cooper, is 
referable to S. crassidentatus (Branson & Mehl) but the holotype is definitely a 
separate species. 
Spathognathodus plumulus plumulus sp. et subsp. nov. 
Plate I, figs. Ia—2c, 5, 6 
DERIVATION OF NAME. From the plume-like anterior blade. 
Diacnosis. Spathognathodid with plume-like anterior blade, denticles of which 
decrease rapidly in size anteriorly from a massive denticle at posterior end of blade. 
Series of lateral nodes present, developed on the outer side only, above the basal 
cavity. 
MATERIAL. 442 specimens : Holotype X 476, Paratypes X 379, X 380, X 381 
(all figured). 
TYPE LOCALITY AND HORIZON. R. Clydach, Nr. Gilwern, K Zone North Crop. 
Sample KL 4. 
RANGE. North Crop KL 1-KL 16, Avon Gorge K 2-Kg I. 
Description. The blade is plume-like in anterior third, consisting of 3 to 6 
denticles, decreasing rapidly in height anteriorly from a tall massive denticle situated 
immediately anterior to the basal cavity. The remainder of the unit is low, consist- 
