VI THE CHICAGO ACADEMY OF SCIENCES. 
outlining their general morphology, their ecological relations and their 
geologic and geographic distribution. A new classification is also pro- 
posed, based on characters of the internal organs as well as upon the 
shells. During the progress of the work several visits have been made 
to Washington and Philadelphia to consult the extensive collections in 
the Smithsonian Institution and in the Academy of Natural Sciences 
of Philadelphia and also to examine and describe the types in these in- 
stitutions. Upward of 100,000 specimens have been examined, cover- 
ing the entire continent of North America. Specimens have been freely 
placed at the writer’s disposal by almost every student of the Mollusca 
in the United States and Canada, and to their generosity, credit is due 
in large measure. A collection embracing the majority of the species, 
and numbering some 20,000 specimens (about 1,500 lots) has been 
built up in the Chicago Academy of Sciences, from which the majority 
of the figures used to illustrate this work have been made. For the 
benefit of future workers, the catalog numbers of these specimens have 
been indicated in the descriptions of the plates. 
ACKNOWLEDGMENTS. 
The success of the present work is largely due to the kind help 
and encouragement of a group of American conchologists who have 
cheerfully given counsel and assistance, besides the loan of collections, 
during the eight years of its preparation. Chief among these the 
author is indebted to Mr. Bryant Walker, of Detroit, Michigan, at 
whose suggestion the work was undertaken; to Dr. Henry A. Pilsbry 
Curator of Mollusks in the Academy of Natural Sciences of Phila- 
delphia, and to Dr. William H. Dall, Curator of Mollusks in the United 
States National Museum, especial acknowledgments are due; my 
thanks are also due to Dr. Paul Bartsch of the National Museum and 
to Mr. E. G. Vanatta of the Philadelphia Academy, for material as- 
sistance while examining specimens in these two institutions. Recog- 
nition is due Mr. Edgar A. Smith of the British Museum, and the late 
Dr. J. F. Whiteaves, of the Canadian Geological Survey, for the loan 
of type material. Special mention should be made of the careful work 
of my assistant, Mr. Frank M. Woodruff, who prepared the excellent 
photographs which accompany this work. Acknowledgment is due the 
Smithsonian Institution for the loan of specimens and for certain 
photographs of type material; to the Academy of Natural Sciences of 
Philadelphia, for the loan of critical material; and to Dr. S. A. Forbes, 
Director Illinois State Laboratory of Natural History, Urbana, IIl., 
tor the loan of the following plates: L, LI, LII, fig. 1, LIV, fig. 1. 
