420 VENERIDiE. 



the convexity is broadly difFusecI, and diminishes in nearly 

 an even ratio at either extremity. The exterior, which is 

 ordinarily of a rulons flesh-colour, generally variegated in 

 the younger examples with amorphous stains of a deeper 

 tint, but which occasionally, though very rarely, is in the 

 northern shells of an uniform chalky white, is rayed with 

 extremely numerous narrow ribs, which near their termi- 

 nation become divided by a groove, but not bifurcated, 

 and are much more closely set in front than beyond the 

 umbonal ridge, behind which they are replaced by most 

 crowded and elevated radiating stria? ; these, as well as the 

 preceding costellse, are concentrically decussated by raised 

 lines. This decussation exhibits itself upon the striye in 

 continuous concentric series of sublunate scales (the last 

 five or six are not affected by it), and forms somewhat 

 tubercular rows of crenro upon the ribs, but is not usually 

 distinct in their interstices, which occasionally are sub- 

 divided by a still narrower costella. The crenre are gene- 

 rally less marked upon the hinder ribs, and in a very rare 

 variety are altogether converted into scales. The northern 

 examples have their ribs in general rather further apart, 

 and somewhat more elevated and square-topped than the 

 southern ones, but never deviate so far from the typical 

 character as to have them remote or much raised. 



The ventral margin is moderately curved, and rises 

 rather the more in front, where it is more or less arcuated. 

 The anterior side is the shorter, and tapers a little at its 

 extremity, which is better rounded below than above. The 

 posterior termination is attenuated, but rounded at the tip. 

 The dorsal edges are moderate in both convexity and slope, 

 the front never being incurved, as in most of the Veneres, 

 and the hinder being almost straight, and scarcely de- 

 cUniiJg as far as the ligament extends. This latter is ex- 



