Z PREFACE. 



varieties of other species by Dr. Binney. In treating these, 

 I have followed the opinions of the latest writers, founded 

 on the advanced state of our knowledge of this depart- 

 ment of science. 



In all doubtful cases, I have given the original descrip- 

 tion and figure of the authors whose opinions conflict, 

 often reserving my own decision till more information has 

 been obtained. I have also endeavored to increase the 

 usefulness of my work by adding many descriptions and 

 figures from rare and generally inaccessible works. 



The measurements of the shells described are the same 

 as adopted by Dr. Pfeiffer, and are given in the millimetre, 

 which is equal to ^ of an inch. I have also followed 

 Dr. Pfeiffer in the systematic arrangement of the various 

 genera. 



The species of the Western Coast are grouped sepa- 

 rately. I have described only those which have actually 

 been found within the limits of the United States, though 

 I have added the names of all the extra limital species 

 found north of Mazatlan. In the second division of the 

 work, I have included the species inhabiting the whole 

 continent of North America, from the boreal regions to 

 the Rio Grande. 



I would take this opportunity of returning thanks to the 

 numerous friends who have aided my studies by furnish- 

 ing me specimens and interesting information regarding 

 the species of their vicinity. I am particularly indebted 

 to my friend Thomas Bland, Esq., of New York, for the 

 use of his most interesting collection of American land 

 shells, as well as for the great advantage I have derived 

 from his valuable assistance during the four years I have 

 devoted to the following pages. To Mrs. Thomas Say, 

 also, I am under very great obligations for the use of her 

 husband's MSS. and many letters from Fenessac and 

 other conchologists. Many of Say's types, preserved in 



