VI PREFACE TO FOURTH EDITION. 



These tables were originally made for my own use. Subsequently I was induced 

 to print them, and they have now expanded to a large volume. "With all the care 

 which has been bestowed on them, I do not present them as a perfect and complete 

 work, and I trust that any omissions and errors may be pointed out to me. 



The Bibliographical List of the Unionidce at the end of the volume will surprise 

 the general student by its enormous extent, comprising, as it does, about eleven 

 hundred references, and yet I am perfectly aware that many works will be found to 

 exist which have never come under my notice. 



Philadelphia, June, 18T0. 



