330 INTRODUCTION TO CONCHOLOGY. 



approximation to each otlier^ as in the genus Cardium and 

 otherSj divaricate in an opposite direction, and become most 

 symmetrically involuted. The bifurcate extension of the 

 ligament is also peculiar. Only five species are at present ^ 

 known : two of similar character, one British or Irish, the 

 other Mediterranean, distinguished from each other by their 

 transverse or globose form, with a variation in the incur- 

 vature of the umboes ; and three of like affinity from the 

 Eastern world, the specific peculiarities of which are deter- 

 mined by variations of form and by the varied development 

 of the concentric ridges. 



The first peculiarity that invites attention, both in the 

 CarditcB and Cypricardm, as distinguished from the beau- 

 tiful cordiform symmetry of the Isocardia, is the great 

 dissimilitude between the anterior and posterior portions of 

 the shell; the former being remarkably short and con- 

 tracted, whilst the latter is always more or less elongated or 

 rounded. Two simple, thick, oblique teeth are conspicuous 

 in one valve of the Cardita, and one of wliich, varying in 

 length according to the elongated proportion of the shell, 

 interlocks, with a single oblique plait, in the opposite 

 valve. It almost seems as if the young mollusk inhabiting 

 the Allied Cardita {C. affinis) took peculiar pleasure in 



