BRITISH AVONIAN CONODONT FAUNAS 137 
which are biconvex in cross-section and have inconspicuous lateral edges. They are 
separated by a great distance and the distal is the smaller of the two. The whole 
anterior aboral surface tapers rapidly towards its distal tip ; although its depth 
remains constant, its width decreases conspicuously. Its distal tip is also flexed 
gently forward. In lateral view the most striking feature of the anterior aboral 
process is that it is so recurved posteriorly that it has an angular protruding junction 
with the base of the main denticle. It is then flexed forwards so that the whole 
appearance of the combined aboral process and anterior denticle is of a feebly sinuous 
line. In outer lateral view the flexure and decrease in width of the anterior aboral 
process are conspicuous features. The lip below the main denticie is also strikingly 
developed in lateral view, the whole inner lateral face of the unit being very feebly 
convex. In aboral view the unit is greatly expanded below the main denticle and 
there is a conspicuously flattened, but sloping, surface which is deeper on the outer 
lateral side. It narrows rapidly towards the posterior bar, which is excavated by a 
relatively deep, rounded groove. The main basal cavity is restricted to the median 
part below the main denticle. The anterior aboral process also has a very wide 
aboral surface, which is only partly excavated, but it narrows rapidly towards a 
pointed distal end and is somewhat twisted in aboral view. It is deflected to make 
an angle of go° in a vertical plane with the main denticle. 
Ligonodina roundyi Hass 
Plate 26, figs. 13a-14b, 16a—c 
1926 Prioniodus sp. A. Roundy : 11, Pl. 4, fig. 9. 
1926 Prioniodus sp. C. Roundy : 11, Pl. 4, fig. 11. 
1953 Ligonodina roundyi Hass : 82-83, Pl. 15, figs. 5-9. 
1956 Ligonodina roundyi Hass: Elias: 126, Pl. V, figs. 10-14. 
1958 Ligonodina voundyi Hass ; Rexroad : 21, Pl. 3, figs. 1-4. 
1961 ?Ligonodina typa (Gunnell) Higgins : 220, Pl. 11, fig. 6. 
Ligonodina voundyi Hass ; Collinson & Druce (in press). 
MATERIAL. 24 Specimens: figured, X 214, X 215, X 216. 
Rance. North Crop 3D 14/15. 
DeEscriPTION. This species of Ligonodina is characterized by a massive and 
elongate main denticle and a strongly developed antero-aboral process. The 
posterior bar is relatively slender in comparison with the proportions of the two 
latter elements. 
The main denticle is greatly elongated, and more or less strongly expanded 
anteriorly and posteriorly at its base. It is sharply pointed and in its distal half it is 
very strongly laterally compressed, the anterior and posterior edges being sharp and 
the lateral faces feebly convex. The curvature of the main denticle is concentrated 
in the proximal third, the remainder being straight. It is also twisted in the vertical 
plane, so that the anterior edge points inwardly ; in its outer aboral portion the main 
denticle is strongly rounded, but the corresponding inner lateral face is flat to feebly 
concave. Along the inner anterior margin a relatively strong and sharp keel is 
