136 BRITISH AVONIAN CONODONT FAUNAS 
strongly recurved in a vertical plane and its proximal aboral surface makes almost a 
right angle with the posterior bar. It is also strongly recurved with respect to the 
aboral surface of the posterior bar. The curvature of the anterior edge of the 
process is in line with the anterior denticle and not with that of the main denticle. 
It makes an angle of about 45° with the posterior bar in a horizontal plane. 
The aboral surface is excavated by a very shallow and rather inconspicuous 
groove, which runs along at least the anterior part of the posterior bar and is con- 
tinuous with a similar cavity below the anterior aboral process. The cavity below 
the main denticle is very narrow and not over-deep. 
REMARKS. This species is distinguished from all other ligonodinids by the form 
and the position of the denticle anterior to the main denticle. In this feature it 
approaches, but does not reach, the typical forms of the genus Magnilaterella 
Rexroad and it is also reminiscent to some extent of some Ordovician species of the 
genus Phragmodus. 
Ligonodina osborni sp. nov. 
Plate 26, figs. Ia—2c 
DERIVATION OF NAME. This species is named in honour of Mr. S. Osborn of the 
Geology Department, University College of Swansea. 
Diacnosis. Ligonodinid with a conspicuous continuously recurved main denticle, 
strongly biconvex in cross-section, with feeble anterior and posterior edges, distal 
portion not preserved in present specimens. Posterior bar of unknown length, 
strongly depressed in vertical plane, bearing on oral surface series of isolated, 
posteriorly inclined denticles, appearing to increase in size posteriorly ; separated by 
distance about equal to their basal width. A short, sinuous, sharply flexed, pointed 
anterior aboral process, bearing 2 isolated denticles ; proximal larger than distal. 
MATERIAL. 6 specimens : Holotype X 212, Paratype X 213 (both figured). 
TYPE LOCALITY AND HORIZON. North Crop. Sample 3D 14/15. 
Rance. North Crop 3D 8-3D 14/15. 
DEscRIPTION. The main denticle is large and strongly recurved, its whole 
anterior edge being strongly convex, and its posterior strongly concave. The distal 
portion is of unknown form but the basal part is biconvex in outline, with only 
inconspicuously developed anterior and posterior edges. It is extended posteriorly 
into a strongly arched posterior bar, the aboral edge of which is concave. Its lateral 
faces are flat or only very feebly convex, and there is a sharp basal ledge developed 
along its preserved length. Its oral surface bears at least 3 isolated, posteriorly 
inclined, denticles, which are biconvex in cross-section, and the third of which is 
larger than the first or second. These are inclined at an angle of about 45° to the 
posterior bar, and are separated by a distance about equal to their basal width. 
The antero-aboral process makes an angle in a vertical plane of about go0° with the 
line of the posterior denticle, so that its denticles point directly inwards and also a 
little posteriorly, because of their recurvature. It bears at least 2 isolated denticles, 
