CONOCARDIUM. 461 



The hinge-line is straight, produced in front, elongated posteriorly, and continuous 

 with the upper margin of the tubular rostrum. The umbones are small, pointed, 

 incurved, contiguous, much cut away behind by the compression of the posterior 

 surface, often carinate : they are remote from the anterior extremity. Passing 

 obliquely downwards and backwards from the umbones to the postero-inferior 

 angle is a well-mai'ked ridge or rounded gibbosity, which separates the lateral 

 from the posterior surface ; a process of shell is often to be found, as a wide 

 flange, extending beyond this ridge, with much the same direction. The lateral 

 surface is convex, the convexity increasing in degree to the extreme posterior 

 margin ; but there is a well-marked narrow sulcus, which passes downwards and 

 slightly forwards. Starting immediately in front of the umbo, and becoming 

 wider and deeper as it passes across the valve, this sulcus shows itself as a well- 

 marked indentation in the lower border. In front, the upper edge of the valve is 

 curved inwards on itself. 



Interior. — Anterior adductor muscle-scar large, deep, bounded behind by a 

 thick oblique ridge. Posterior adductor scar shallow and almost obsolete, placed 

 on the posterior surface remote from the margin. Cavity of the shell smooth. 

 The margin is roughened in front, but soon shows several interrupted, distinct, 

 short ribs, which become smaller and smoother (? worn) as they pass backwards. 

 Immediately posterior to the byssal groove the edge of the valve is guarded by 

 alternate, elevated, pointed teeth, and corresponding sockets, which become 

 slightly smaller as they pass backwards ; these appear at the extreme postero- 

 inferior angle of the left valve to be a single rounded socket of larger size than 

 any of the others. The posterior margin is also marked on its inner surface with 

 smaller, but sharp, narrow, pointed teeth, alternate with corresponding sockets, 

 diminishing in size from below upwards. Immediately below the umbones, on a 

 plane external to their apices, is a flattened hollowed plate, the anterior end of 

 which terminates abruptly ; but its base is continuous with the ridge posterior to 

 the anterior adductor scar. This plate supports the ligament, which is situated 

 thus partly internal and partly external. There is no evidence of the presence of 

 hinge-teeth in the ordinary sense of the term. 



Exterior. — The surface is covered with radiating ribs, often much obscured by 

 shell-growth, but very apparent when the outer layer of shell is removed. The 

 posterior surface is ornamented in the same way by curved ribs and narrow sulci, 

 closer together than on the lateral surface. In the perfect condition the surface 

 is almost smooth, with fine, close, almost microscopic lines of growth, occasionally 

 interrupted by deeper grooves. 



Shell very thick. The inner surface is smooth and compact, and is separated 

 from the outer, also nearly smooth, by a numerous series of radiating cellular 

 ribs, divided into spaces transversely by thin lamellas of shell. These ribs are 



