176 MYAD^. 



feel ourselves justified iu terming it, since, altlioug-li there is 

 a slight flattening of the surface between it and the front 

 extremity, there is no defined anterior limit to the so-called 

 ridge. The exterior is of an ashy buff colour, and utterly 

 devoid of lustre ; the interior is paler or darker ochraceous, 

 and moderately glossy. 



The ventral margin is much incurved in the middle, 

 swelling out at the extremities, and rapidly and obliquely 

 ascending in front. The dorsal margin for the most part 

 runs nearly parallel with the ventral, scarcely declining at 

 all behind, (where it is incurved, and where the dorsal sur- 

 face is flattened,) but in front forming an uniform convex 

 or arcuated sweep with the anterior outline, by which that 

 extremity is distinctly rounded, yet not symmetrically so, 

 owing to the trifling declination of the upper portion, com- 

 pared with the longer and more abrupt curve of the lower 

 one. The hinder side is nearly twice as long as the front, 

 and is very bluntly biangulated at its termination, which 

 is broad and not at all tapering : the hinder edge is nearly 

 straight, and inclines a little obliquely outward from its 

 dorsal commencement. The mnbones project but slightly 

 above the cardinal edge, being rather flattened above, and 

 the beaks are very small and lean almost directly in- 

 wards. The ligament is broad and rather prominent, but 

 not very elongated, and in the few shells we have seen 

 is of a dark colour. Notwithstanding the solidity of the 

 shell, the edges are acute and simple, and for the most 

 part incline outward. The hinge margin is very broad, 

 and is merely provided in each valve with a remarkably 

 small apical tooth, and an adjacent receptacle for that of 

 the opposite one : these occupy but little of the hinge 

 plate, which is there shallowly excavated, with its lower 

 edge, to which they scarcely reach half way, minutely 



