406 VENERIDiE. 



The general form is subquailrate and suLorbicular, and 

 the profundity, which varies from actually ventricose to 

 but moderately convex, is always more marked behind 

 than in front. The valves are extremely inequilateral, 

 strong (not unfrequently thick and heavy), opaque, but 

 moderately glossy, whitish, pale ferruginous brown, or 

 cream coloured, and occasionally painted with from one 

 to three rosy-red more or less interrupted rays, Avhich are 

 not particularly broad, and are often partially indistinct, 

 the one which runs from behind the beaks to the lower 

 posterior corner being least liable to become obsolete. The 

 exterior is covered with very numerous concentric laminar 

 simple (not fimbriated) much elevated jjlates, which are 

 more solid depressed and closely set in front, bend to- 

 wards the beaks in the middle, and become erect, or even 

 deflected, behind. Their interstices are not decussated by 

 any radiating striae or costellse, but are often subdivided by 

 irregular concentric strise or incipient lamella). In the 

 young, and upon the umbonal region of the adult, these 

 plates are rather thin, but usually become more or less 

 solid towards the ventral margin. This latter is decidedly 

 arcuated, and rises the more in front. The anterior side is 

 not merely very short, — for it occupies ordinarily but one- 

 fourth, and frequently a still less proportion of the entire 

 length, — but is likewise much narrower than the hinder ex- 

 tremity, its extent being diminished as well by the ascent 

 of the lower margin, as by the more or less rapid declina- 

 tion of the upper one. The general inclination of the front 

 dorsal edge is, from the jiouting of the lips of the lunule 

 (which latter is large, heart-shaped, and strongly defined 

 by being sunk below the general level at its margin), nearly 

 rectilinear; the front extremity is narrowed, and unsym- 

 inetrically rounded. The hinder termination is broad and 



