PREFACE. 



The following pages contain the results of several years' 

 study of one of the most interesting and difficult branches of 

 American Conchology. My MS. was completed in 1865, 

 and I find, upon freshly taking up the subject, that I am 

 inclined to question many of the conclusions at which I had 

 then arrived. A more enlarged acquaintance with fresh-water 

 shells convinces me that a much greater reduction of the 

 number of species than I have attempted must eventually be 

 made ; but until the prolific waters of the southern states 

 have been systematically explored and a great collection of 

 specimens obtained, which shall represent every portion of 

 those streams and include as many transitional forms as can 

 be procured, a definitive monograph of our Melanians cannot 

 be written. I am indebted to several kind friends for assist- 

 ance in preparing this w^ork ; first of all, to Dr. Isaac Lea, 

 who not only gave me constant access to his noble collection, 

 but on many occasions aided me by comparing specimens and 

 elucidating knotty questions in synonj'my. Mr. John G. 

 Anthony, Prof. S. S. Haldeman and the late Dr. Aug. A. 

 Gould, with great liberality, sent to me their tj'pes ; and in 

 these collections and that of the Academy of Natural Sciences 

 of Philadelphia, I also found t3'pes of man}'^ of the species 

 described by Say and Conrad. Most of my synonymy is 

 derived from the direct comparison of these t3'pical shells, and 

 to this extent I believe my work will prove to be reliable. 



G. W. T., Jr. 

 November, 1873. 



(V) 



