INTRODUCTION. 



This systematic arrangement and synonymy was commu- 

 nicated to the American Philosophical Society in July last 

 nearly in the form in which it is now printed. It was 

 undertaken purely with the view and in the hope of clear- 

 ing away the difficulties which had incumbered one of the 

 most interesting families of the Mollusca. In this attempt 

 the author met, while pursuing his task, with obstructions 

 and difficulties which he little anticipated at its commence- 

 ment. The want of some of the books of reference, and th.e 

 confusion which reigned throughout many of them, some- 

 times presented obstacles which seemed almost insurmount- 

 able. In attempting to establish the synonymy, he has 

 endeavoured to render the strictest justice, and if in any case 

 it is found he has failed to do this, it will be a matter of sin- 

 cere regret to him. 



In the following tables there will be found in the family 

 300 recent species as admitted, 22 as doubtful, and 22 fossil ; 

 in all 344. 



Of the sub-gciius Unio there are 22G species in a recent 

 state, and 14 which I have not been able to admit as certain ; 

 of fossil species 21. 



