CARDIUM. 17 



after forming a slight angle with the dorsal margin, slopes 

 somewhat obliquely, and makes with the ventral a rounded- 

 off inferior angle to the hinder side, which is occasionally 

 much produced, and tapering at its extremity, occasionally 

 broad, subbiangulated, and scarcely longer than the front 

 one. The umbones are greatly prominent, and the beaks 

 very acute, and much inflected. 



The ligament is very large, and highly projecting. The 

 interior is white, with a dark-chestnut stain at the posterior 

 extremity, and often a still deeper one on the hinder hinge- 

 margin. The lateral teeth are strong, much elevated, not 

 very remote, and nearly equidistant. 



If we take an earlier stage of this bivalve, for instance, an 

 example not exceeding three-fourths of an inch in length, 

 we shall perceive many and important differences, but such 

 as may be traced up in regular gradation to the mature 

 shell. The shape is then suborbicular, the sides equal or 

 nearly so, there being no inferior hinder angle ; an upper pos- 

 terior subangulation is, however, distinctly visible, and the 

 hinder outline is more or less convex, and scarcely oblique. 

 The ribs are rather more elevated, and all of them adorned 

 with the linear scales (which, however, are very easily worn 

 away), and flexuous livid zones occasionally make their 

 appearance on the umbonal region. 



Although apparently differing from edule quite as essen- 

 tially as many of the more recently established species of 

 Cardium do from each other ; yet, as we are able to appre- 

 ciate in the rusticum of British writers the modifications of 

 form, sculpture, and colouring, produced by a greater or 

 lesser admixture of fresh water with the saline element, we 

 venture not to describe it as specifically distinct, but hold it 

 rather as a remarkably aberrant variety of the Common 

 Cockle. 



vol. n. o 



