CONOCARDIUM HIBERNICUM. 469 



CoNOCAEDiUM QIGANTEUM, Bigshy, 1878. Ibid., p. 303. 



— TBiGONALE, Bigshy, 1878. Ibid., p. 303. 



— HiBEENicuM, de Koiiinck, 1885. Ann. Mus. Eoy. Hist. Nat. 



Belg., vol. xi, p. 101, pi. xviii, figs. 7, 8. 



— — Etheridge, 1888. Brit. Foss., pt. 1, PalsBozoic, 



p. 281. 



— GIGANTECM, Etheridge, 1888. Ibid., p. 281. 



— TKiGONALE, Etheridge, 1888. Ibid., p. 281. 



Specific Characters. — Shell very inequilateral, very short antero-posteriorly, 

 deep ; of only moderate size, trigonal, horse-lioof shaped, acutely ridged, 

 abruptly truncate, and much adpressed posteriorly. Posterior end obsolete, with 

 an expanded cordate flattened posterior surface. 



The anterior end is bluntly conical, rapidly narrowed from above downwards 

 at the expense of its lower border, and rapidly compressed, but gaping ; separated 

 from the gibbose portion of the valve by a broad well-marked sulcus. The 

 inferior border slopes rapidly downwards in a sinuous line, convex at each 

 extremity, but concave in its centre, terminating abruptly at the postero-inferior 

 angle. The hinge-line is very short, straight, continuous posteriorly with the 

 upper margin of the long rostrum. The umbones are small, incurved, gibbose in 

 front, but abruptly truncate behind. Passing vertically downwards from the 

 apex of the umbo is a strong angular keel, which forms the border between the 

 lateral and posterior surfaces. To it is attached a broad shelly expansion, which 

 forms a flange all round the posterior surface of the shell, nearly equalling in 

 depth that of the shell itself, and curved backwards so as to enlarge considerably 

 the posterior surface, which is made by it much more concave, the concavity 

 looking backwards. The posterior surface is excentrically concavo-convex ; 

 externally there is a smooth zone, slightly convex; further inwards the surface 

 becomes concave except along the margin, which is raised and forms an elevated 

 line from the lower margin of the rostrum to the lower angle. This raised line 

 itself is concave, and still more internally towards the upper part of the surface 

 becomes smooth and elevated, forming the base of the rostrum, which is tubular, 

 of moderate diameter, and of uncertain length. 



Interior. — No details have been observed in this species. 



Exterior. — The surface of the shell is nearly smooth, but marked with fine con- 

 centric lines and striae of growth, and fine radiating lines. The majority of 

 specimens have the outer layer of shell destroyed, and show series of radiating ribs 

 of various sizes, according to the depth of the layer; the lowest ribbed layer con- 

 sists of strong, well-separated, radiating flattened ribs. On the posterior surface 

 the concave portion is finely and excentrically ribbed. The flange is very finely 

 striate radially. Shell thick, with several layers. 



