4 PREFACE. 
matids (Melanians). The latter will form the 
subject of a future memoir. 
Prof. 8. 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 Turbide and Physade, 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 
