8 CARDIAD^. 



coming of a more elongated outline with age. The valves 

 are occasionally very solid, yet quite as frequently, but 

 moderately strong, and not particularly heavy : they are 

 opaque and tumid, destitute of lustre, and range from 

 whitish (usually with squalid zones) to brownish rust 

 colour. The surface is radiated with eighteen or nineteen 

 strong and elevated ribs, which, emanating from the in- 

 curved beaks, squarely dentate the ventral margin at their 

 termination. These ribs, which are well raised, and some- 

 what square-topped, bristle with crowded spines, which, 

 planted nearly in the centre of them, are connected at their 

 bases with each other, and assume a different look and 

 shape according to the stages of growth and their various 

 positions upon the shell. They are never much produced, 

 and are generally indeed decidedly short : in the young, 

 they are so compressed laterally as to form a coarsely ser- 

 rated carina upon the ribs, which is consequently their 

 appearance upon the umbonal region of the adult ; they 

 next become almost separated pyramidal sharp-pointed 

 spines, of which the anterior rows are suddenly bent back- 

 wards, and their upper surfaces a little grooved out ; 

 finally the sharp ends gradually wear off, and their bases 

 become solid, and no longer compressed. These prickles, 

 which are generally of a paler hue than the ground colour, 

 are coarser, more remote, and more spatulate in front ; 

 smaller, sharper, and more clustered behind. The inter- 

 stices of the costte are about equal to them in breadth, and 

 are coarsely irregularly and somewhat flexuously rough- 

 ened by concentrically disposed elevated wrinkles, whose 

 peculiarities are most marked at the extremities of the 

 shell, where the ribs greatly diminish in elevation, and their 

 proportion to the intervening sulci is no longer observed. 

 The ventral margin, by obliquely ascending in front, 



