PEEFACE TO FOURTH EDITION. 



It is now eighteen years since the last edition of this "Synopsis" was piiblished. 

 The advancement of the knowledge of JSTatural History has been accelerated during 

 that period by the labors of many men of science, the younger of whom have 

 embraced the study with great earnestness and energy. The effect of this progress 

 has been felt very sensibly in that interesting group of Mollusca which embraces the 

 Family Unionidce. Consequently the additions of species and synonyms have been 

 largely increased, resulting, in fact, in more than doubling the lists of the last edition. 

 The labor required for this portion of the work has been very great. 



The same principles of arrangement of the tabular system which were used in 

 the previous editions have been retained in this. After the experience of thirty-four 

 years since the first edition was issued, I have had no i-eason to believe that any 

 better arrangement could be devised in the present state of our knowledge. The 

 tables have therefore been extended to the present time, and have been so much en- 

 larged with new matter as almost to entitle the work to be considered a new one. 

 This has been done with a faithful desire to recognize every fellow-laborer in this 

 group, and to render to each one full justice. Some may have been omitted, for 

 want of access to their works — none intentionally. 



The present edition is greatly improved and enlarged by additions to the geo- 

 graphical distribution tables, and by the habitat of each of the species ; and the 

 Index referring to the tables of species and synonymy has had the exact references 

 fully made to the works where they were originally announced. These two very 

 important additions to the previous editions will facilitate the use of reference to all 

 the works on the subject which this "Synopsis" embraces. 



