OTRODUCTIO^. 



OBJECTS. 



The want of a complete and consistent list of the principal 

 subdivisions of the mollusks having been experienced for some 

 time, and such a list being at length imperatively needed for the 

 arrangement of the collections of the Smithsonian Institution, the 

 present arrangement has been compiled for that purpose. It must 

 be considered simply as a provisional list, embracing the results 

 of the most recent and approved researches into the systematic 

 relations and anatomy of those animals, but from which innova- 

 tions and peculiar views, affecting materially the classification, 

 have been excluded. The only merit which is claimed for it is 

 the embodiment and co-ordination, it is hoped in a tolerably con- 

 sistent form, of the taxonomic results of the information scattered 

 through many volumes. There will doubtless be much diversity 

 of opinion respecting the relative value of certain groups, as well 

 as of the characters themselves whose modifications have been 

 used for the limitations of the groups, and the author will not 

 disguise that he himself entertains much doubt respecting certain 

 groups and relationships preserved in the arrangement. It has 

 seemed advisable, however, to provisionally adopt the opinions 

 of those who have most thoroughly investigated the different 

 groups rather than to introduce innovations based on hypothe- 

 tical considerations, and which would be perhaps found to be 

 liable to as many objections as those adopted 



But although, from the very nature and extent of the subject, 

 the present arrangement is a compilation, it nevertheless is like- 

 wise the result of researches undertaken by the author with more 

 or less assiduity for a number of years, and, as a whole, it offers 

 a considerable number of deviations from any classification 



( vii) 



