GENERA OF SHELLS. 29 



The venericardiae seem to connect the conchae with 

 the cardiacea ; their radiating ribs give them perfectly 

 the appearance of cardium, and the hinge would re- 

 semble the venus if it had a third diverging tooth on 

 each valve. It appears to differ from cardita merely 

 by wanting the lunular tooth, two oblique teeth repre- 

 senting the lateral tooth of cardita, which is always 

 channelled. They are chiefly fossil. 



V. planicosta, petuncularis, imbricata,australis, acuticosta, 

 mitis, senilis, laBvicosta, concentrica, decussata, elegans 

 tridentata. 



CARDIACEA. 



Cardinal teeth irregular, either in their form or situation ; 



or generally accompanied by one or two lateral teeth. 

 The cardiacea are ventricose shells ; almost all are 

 furnished with radiating, longitudinal ribs, and when 

 viewed at the fore part are heart shaped. They are 

 equi valve, regular, and sometimes gaping. 



CARDIUM. Shell equivalve, subcordiform, 

 beaks protuberant, valves dentated or plaited on 

 the internal edge. Hinge with four teeth on 

 each valve, the two cardinal approximate and 

 oblique, the two lateral articulating crosswise 

 with their opponents ; two lateral, remote, en- 

 tering. 



The prominent, cordate beaks of this genus, are very 

 remarkable. The convex side of the valves is gen- 

 erally furnished with longitudinal ribs, more or less 

 prominent, frequently striated, imbricate or spinous; 

 but the interior is smooth, and only furrowed towards 

 the margin. The ligament is external and very short; 

 there are two, faint, muscular impressions. They are 



