INTRODUCTION. 



The design of this catalogue is simply to facilitate exchanges with corres- 

 pondents. The author will be happy to exchange with any persons to whom it 

 may be sent by himself or by his friends. Species of North American, or of Eu- 

 ropean shells, which are not named in this catalogue, are particularly desired, 

 and in return for them, any person, who will forward a list of all the species 

 herein named which are wanting in his collection, may expect an equal num- 

 ber of such species. 



Of those species herein named which are not common, fine specimens will be 

 received, and an equal number of species will be returned; but in this case if is rec- 

 ommended that a copy of this catalogue be first returned to the author with a mark 

 prefixed to the names of the species which it is proposed to send, in order to a- 

 void the exchange of useless duplicates. 



Part III will contain a catalogue of the remainder of the collection; but on ac- 

 count of the number of species yet undetermined, the printing of it is deferred 

 to some future time. Meanwhile any species, from countries oth&r than the above 

 named, which are not generally distributed in collections, will be received in ex- 

 change. 



The most just rules of exchange seem torequire, in general, the return of an equal 

 number of species and, in the total, of specimens, and the number of specimens 

 not to be very unequally distributed among the species. The latter object is best 

 secured by affixing a limit to the number of specimens in the species, as at 20. 

 Very rare and valuable shells however should be made the subject of special re- 

 mark. As some correspondents regard more the number of species, others — the 

 number of duplicates, and others — the rarity and commercial value of the speci- 

 mens,attention to these simple principles may be necessary to avoiddisappointment. 



The writer takes pleasure in bearing testimony to the almost universal preva- 

 lence, among those naturalists who have honored him with their correspondence, 

 of such sentiments of honor, as have secured a fairand often a most'liberal return 

 for specimens which have been forwarded. Only one solitary case of 'repudia- 

 tion' on the part of a wffl^MraZ/snias occurred in his experience, and since in this 

 case the naturalist was also a dealer, it can scarcely be considered as an excep- 

 tion to the general rule. 



In the names of the species and of the authors of the names, the writer has en- 

 deavored to observe the law of priority as far as permitted by the limited means 



