X PREFACE 



but the illustrations of them appear now, to complete the data needed by the 

 student, as well as figures of a few other interesting forms from similar material 

 received too late to be utilized in the body of the work. 



In the years which have elapsed since this Memoir was begun the subject 

 of zoological nomenclature has been much discussed and the general consensus 

 of opinion seems to trend towards the acceptance of names for which no diag- 

 nosis was originally supplied, provided the species cited under them are identi- 

 fiable. This change from the British Association rules of 1842 is responsible 

 for much unnecessary overturning of formerly accepted names with no visible 

 benefit to science, but since it appears to express the will of the majority it 

 seems useless to oppose it, and in Parts IV.-VI. it has been complied with 

 except in the case of the anonymous auctioneer's catalogue known as the 

 " Museum Calonnianum." This compilation from a manuscript of Hwass, 

 edited by Da Costa, and printed for the auctioneer, George Humphrey, has 

 usually been credited to the latter. I confess my desire to settle the nomen- 

 clature on a firm basis, though great, has not been equal to the acceptance of 

 these anonymous, undefined, worthless names, which would involve the loss 

 of much that is most fundamental in the nomenclature of moUusks. I still 

 hope that the common-sense of naturalists will find a wa}'' — if necessary, an 

 arbitrary way — to eliminate this publication from authorized sources of nomen- 

 clature. The " Museum Boltenianum" stands on a different footing, and the 

 principal change which its acceptance involves in the earlier part of this work 

 is the substitution of the name Btisycon for the more familiar Fulgur. 



Acknowledgments are due not only to the Wagner Institute of Science and 

 its secretary, Mr. Joseph Willcox, for cordial cooperation and sympathy in the 

 work exemplified in this Memoir, but also to the United States Geological Survey 

 and its directors, the late Major J. W. Powell and the Hon. Charles D. Wal- 

 cott, by whose permission it was carried on, and to Dr. Frank Burns and Mr. 

 T. Wayland Vaughan, of the staff, for hearty cooperation ; to the Smithsonian 

 Institution and National Museum by the secretary and ex officio director. Dr. 

 S. P. Langley, for the use of collections, library, and other facilities to an 

 extent only limited by the needs of the occasion ; to Professor W. B. Clark, 

 director of the Maryland Geological Survey, for the loan of type specimens for 

 comparison ; to Dr. H. A. Pilsbry and other officers of the Academy of Natural 

 Sciences for free access to the collections and other courtesies ; to numerous 

 friends and correspondents at home and abroad, among whom should especially 

 be mentioned the Hon. T. H. Aldrich, of Birmingham, Alabama ; Professor 

 Eugene A. Smith, of Tuscaloosa, Alabama ; Dr. L. T. Chamberlain, of New 



