88 BRITISH AVONIAN CONODONT FAUNAS 
of convergent evolution, since the ancestors of C. gilwernensis were of spathogna- 
thodid stock. They may have been functionally similar. 
Clydagnathus unicornis sp. nov. 
Plate 2, figs. 2a—3d, 5a, b 
DERIVATION OF NAME. From the single large denticle on the blade. 
DiaGnosis. Clydagnathid with restricted anterior lateral blade, consisting 
essentially of one large denticle ; median trough opens anteriorly ; basal cavity 
elongated and symmetrical. 
MATERIAL. 25 specimens : Holotype X 79, Paratypes X 80, X 81 (all figured). 
TYPE LOCALITY AND HORIZON. R. Clydach, Nr. Gilwern, Lower Z Zone. North 
Crop. Sample Z 2A. 
RANGE. North Crop ZL 4-ZLA 27, Avon Gorge Z I. 
DEscRIPTION. The unit is asymmetrical, the blade always occurring on the right 
side viewed from the posterior, and consisting essentially of one large denticle, sub- 
triangular in shape and posteriorly inclined. The tip of the denticle is sharply 
pointed and the anterior edge convex, forming a continuous curve with the spatulate 
antero-aboral margin of the blade. The anterior face of this denticle occasionally 
bears 2 or 3 small denticles which are fused with it, so that only thin blunt apices 
are visible. 
The platform is uneven, consisting of two rows of low irregular marginal nodes, 
separated by a very shallow, but rather wide, medial trough. A short carina may 
sometimes be present in the posterior extremity. The platform walls are deep, and 
on the basal faces are disrupted by the flaring of the lips of the basal cavity. In 
lateral view, the marginal nodes are sharply pointed in smaller specimens, but blunt 
or flat in larger specimens. 
In aboral view the unit is excavated, the cavity being asymmetrical, expanded 
more on the inner side than the outer. The cavity runs posteriorly, becoming 
narrower and shallower towards the posterior termination. It is grooved along its 
whole length. 
REMARKS. This species probably arose from Clydagnathus cavusformis by fusion 
of the anteriormost blade denticles to give an essentially unidenticulate blade, and 
by the elongation of the cavity, rather than the laterally expanded cavity of C. 
cavusforms. 
Clydagnathus sp. nov. A 
Plate 2, figs. 4a—d 
MATERIAL. 2 Specimens : figured, X 82. 
RANGE. North Crop KL 7-KL 13. 
