68 C A R D I U M. 



is usually convex, but sometimes much compressed, 

 keeled or angular. The ribs and furrows of the 

 two valves are so disposed as to alternate at the 

 margin, and to lock accurately and firn^ly into 

 each other. Although a variety of shells are as 

 lustly entitled to the aj)pellation xaphioc, or hecirl- 

 like, as those which constitute the genus Cardium, 

 still the latter all possess this character, and are 

 distinguished from others by their hinges. 



In one species, C. echinatum^ a circumstance is 

 observable, which is altogether anomalous in the 

 structure of either bivalves or univalves ; the spines, 

 which are pointed on the anterior and flattened 

 on the posterior side of the valve, have their open- 

 ing or suture situated towards the apex, and not as 

 in other instances towards the lower margin. 

 There is doubtless some singular construction of the 

 animal which effects this formation of the spines ; 

 but for what purpose the shell is made to differ 

 from its congeners, such as C. SocanUa, it is not 

 easy to determine. 



