54 CARDIUM. 



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 firmly into 

 each other. Although a variety of shells are as 

 justly entitled to the appellation Ttagdia, from being 

 heart-like, as those which constitute the genus Car- 

 dium, still the latter all possess this character, and 

 are distinguished from others by their hinges. 



In two species, C. echinatum and aculeatum, 

 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 opening or suture situated to- 

 wards the apex, and not as in other instances to- 

 wards the lower margin. There is doubtless some 

 singular construction of the animal which effects 

 this formation of the spines : but for what pur- 

 pose these shells are made to differ from their con- 

 geners, such as C. Isocardia, it is not easy to 

 determine. 



