76 



possessing carinated or angular umbones, and the pos- 

 terior side being often much larger than the anterior. 



In the second division, La Marck has placed the 

 Cardium cardissa, and similar species of Cardia, having 

 their valves angularly flattened, and being, when clos- 

 ed, in the shape of a compressed heart; forming a re- 

 markable distinction from other bivalve shells, in which 

 the depression is always in the opposite direction. 



It is difficult to imagine that these species are inhab- 

 ited by the same animal that constructed the Cardium 

 costatum, so amazingly do they differ from it in form. 

 La Marck, however, does not make any observation on 

 the subject, and has not even formed a separate divi- 

 sion of them. 



No particular Angle at the Umbones, and the anterior 

 side at least as large as the posterior. 



Cardium costatum Cardium Brasilianum 



ludicum apertuni 



ringens papyraceum 



Asiaticum bullatum 



tenuicostatum ciliare 



fimbriatum echinatum 



