INTRODUCTION. 



I 



In tills sevontli volume I have continued the publication of American as well a> exotic species 

 of the family UnionidiB ; and it will be observed, from the dates of their original publication in the 

 Proceedings of the Academy of Natural Sciences, that the discovery of new species has been ex- 

 ceedingly rapid. I may cite more particularly those from Georgia, in which State there seems to 

 exist a much greater number of species than in any other of the United States. Indeed, it seems 

 to be the Zoological centre of the family. To many intelligent and ardent students of natural 

 history in that State I am greatly indebted for the advantages of examining and describing the 

 new forms which they have discovered. Other kind friends in Tennessee, Alabama^ Missis.sippi, 

 &c., have also contributed to the development of this branch of Natural Histoi-y. To all I return 

 my sincere thanks for their contributions to this and other volumes ; and I must not forget my 

 obligations to friends abroad, who have so kindly and liberally sent their specimens to me across 

 the Atlantic for my examination and use. In all cases these favors are acknowledged in the 

 several descriptions under their proper heads. 



I have often been asked as to the number of the species of Unionkhe in the United States, without 

 being able to answer with any degree of precision. Recently, at the request of the officers of tlie 

 Smithsonian Institution, I have made out a list to be published by that institution, and I found the 

 number to be greater than I had supposed, viz., about 580, which are thus divided : 

 Unio, ...... 465 species. 



Margaritana, 26 " 



Anodonta, 59 " 



550 

 To these might be added new species in I 

 my possession, but not yet described, j 



580 

 And I cannot doubt that at least 100 new species will be found to exist in our fresli-water rivers, 

 lakes and pools, not yet examined. 



From Mexico, Honduras and Central America there have been described 29 species of Unh 

 and 8 of Anodonta, and from Canada 1 Unto. The whole number known to inhabit North 

 America is, therefore, 618; but it must be remembered that Mexico, Honduras and Central 

 America have been but little examined for these molluscs. 



As my papers have within the last two or three years been given mainly to the Georgian species, 

 I think it will not be uninteresting to those who study these molluscs, to set down the number 



