466 CYrRiNiDiE. 



always profomid and incurved in front. The anterior 

 extremity is narrowed and rounded ; the posterior termina- 

 tion is obtuse, but displays a more or less manifest ten- 

 dency to biangulation. The beaks are acute and inclined, 

 and the umbones remarkably prominent. The dorsal im- 

 pressions, but especially the hinder one, are large, and not 

 very profound ; the lunule is lanceolate or ovato-lanceolate. 

 The ligament, which is, as usual, imbedded, is rather broad, 

 enlarged at its termination, and not much depressed ; it 

 occupies two-fifths of the lozenge, and is yellow, or fulvous 

 brown. The internal surface is white, and its margin per- 

 fectly entire. The hinge consists in the left valve of two 

 rather narrow slightly divergent simple teeth, with a 

 cavity between them for the reception of the large solid 

 central simple tooth of the right valve, which has, besides, 

 an almost rudimentary one running nearly parallel with, 

 and adjacent to, the hinder dorsal margin. The front 

 extremity of the hinge margin of the latter valve frequently 

 also assumes the appearance of a distant lateral tooth. 



A remarkable variety has been taken in North Britain, 

 which demands especial notice, approximating as it does 

 to the shape of sulcata^ from which, however, the in- 

 tegrity of its solid margin, the smallness of its size com- 

 pared with its solidity, and the closeness, depression, and 

 narrowness of its ribs, suffice to distinguish it. It is longer 

 in proportion to its breadth, than in the type, and has 

 fewer and broader ribs ; its front extremity not unfre- 

 quently projects so that the anterior side is equal in length 

 to the posterior. This variety, however, appears to unite 

 with the type by almost imperceptible gradations. We 

 may remark, that the smooth variety is always the more 

 compressed one. 



The animal has a plain edged mantle, with sessile si- 



