4 PREFACE. 



matidoe (Melanians). The latter will form the 

 subject of a future memoir. 



Prof S. S. Haldeman having liberally pre- 

 sented to the Conchological Section of the 

 Academy of Natural Sciences of Philadelphia 

 the remaining stock of his magnificent mono- 

 graphs on Turbidas and Physadse, the Section 

 has determined to re-issue that work as one of 

 its series of publications, and has confided to 

 me the duty of preparing a continuation of it, 

 to include all the species published during the 

 past twenty-five years, as well as any changes 

 that may have occurred in the nomenclature 

 of Prof. Haldeman's species during the same 

 period. 



For the reasons fully stated in the preface 

 to my work on Terrestrial Mollusca, I have 

 avoided as much as possible the use of techni- 

 cal language ; and I have also endeavored to 

 render the descriptions of the species as con- 

 cise as possible, by omitting to refer to charac- 

 ters which are common to all the species of a 

 genus or higher group. In fact, my principal 

 aim has been to make myself thoroughly un- 

 derstood not only by scientific men, but also 



