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 Tia^hta, from being 
heart-VjikQ, 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. 
