VENERUPIS. 157 



the interior is of a similar paleness to the exterior, but is 

 always stained with purple or brown on the hinder hinge 

 margin, and is usually tinged or dyed with similar colours 

 near its posterior extremity. It never attains any consi- 

 derable size. Its surface is more or less convex, at times 

 even ventricose, and is totally devoid of lustre. Its solidity 

 is extremely variable, it being sometimes decidedly thick 

 and strong, but quite as often thin and fragile. The sides 

 are extremely unequal, the front occupying ordinarily about 

 one-fourth the length of the shell. The surface is adorned 

 with thin-edged concentric lamellse, which vary in approxi- 

 mation from distant to rather close-set, (the commoner form 

 appears to be rather remote,) and in elevation from de- 

 pressed to erect. These have a decided tendency to enlarge 

 posteriorly, and are fimbriated by the very crowded 

 radiating striae, which become elevated in the interstices. 

 The front extremity is always a little narrowed at its 

 termination by the very decided slope of its upper or dorsal 

 margin, and the ascent of its lower one ; which latter, 

 although adapting itself to the circumstances of its dwelling- 

 place, is typically rather convex and a little retuse near the 

 middle, as an examination of the direction of the upper 

 or undisturbed lamellse will demonstrate. The hinder ex- 

 tremity is always more or less biangulated, and sometimes 

 a little attenuated : the hinder dorsal margin is straight, 

 convex, or even incurved, but never much declining. The 

 beaks are curved forward, and are not preceded by a 

 lunule ; the ligament is small, sunken, and narrow. The 

 hiage consists of three primary diverging teeth in each 

 valve ; the central strong, and bifid or even bipartite, the 

 front in one, and the hinder in the other valve, almost 

 equally as large, but not always so evidently bifid, whilst 

 the third in each is simple, narrow, and at times almost 



