IV 



111, from the pen of Dr. Wm. Stimpsou, was published 

 in August, 1865. 



Of the two remaining monographs, Part IV is given in 

 the following pages, as prepared by Mr. G. W. Tryon, Jr., 

 and Avill, it is hoped, tend to facilitate the study of a very 

 intricate group, little understood. No special arrange- 

 ment has been made by the Institution in reference to a 

 monograph of the Unionidm (which would form a Part 

 VI) since the many illustrated papers and synopses of 

 the group, published by Mr. Isaac Lea iu the Memoirs 

 of the Academy of Natural Sciences, and of the American 

 Philosophical Society, as well as printed privately, ren- 

 der this less necessary. The present work by Mr. 

 Tryon, therefore, completes the series of works on "Land 

 and Fresh-water Shells of North America," as originally 

 contemplated by the Institution. 



JOSEPH HENRY, 



Secretary Smithsonian Institution. 



Smithsonian Institution, 



Washington, December, 1873. 



