INTRODUCTION. j x 



It is much to be regretted, however, that the descriptions of animals 

 were limited to their external characters, and that dissections and 

 delineations of their anatomical structure had not also been directed ; 

 especially in the case of those genera in which the structure of the 

 soft parts is still unknown. A very large supply of mollusks, in 

 spirit, is preserved in the collection, and would afford ample materials 

 for future zoological investigations concerning this class of animals. 



As this work was intended to be purely descriptive, no generaliza- 

 tions were expected. A few points, however, have presented them- 

 selves so obviously, in examining the specimens and memoranda of 

 the shell collection, that it would seem proper to advert to them. 



The doctrine of distinct zoological regions evidently appertains to 

 the mollusks, and is well illustrated by them. In nearly every work, 

 containing any considerable catalogue of shells, the same species will 

 be found quoted as being found in widely distant regions, in different 

 oceans, and even on opposite sides of the globe. The many thousand 

 localities carefully noted on the records of the Expedition, go to prove 

 beyond dispute, that no such random or wide-spread distribution 

 obtains. The error has arisen from two principal causes. One is, 

 that reliable notes of localities have not been taken. A voyage is 

 made to the Sandwich Islands, and all the shells brought home by the 

 vessel are said to be shells from the Sandwich Islands, though thev 



' / 



may have been obtained at California, the Society Islands, New 

 Zealand, and, perhaps, half a dozen other places quite as remote from 

 each other. A sea captain purchases a collection at Calcutta or Val- 

 paraiso, for his friends at home ; and all the shells are marked as 

 denizens of the port where they were purchased, though they might 

 not have lived within thousands of miles. Purchased shells cannot 

 be relied on for localities ; for this end a shell must have been found 

 containing the animal, or else dredged, or picked up on the shore, and 

 labelled accordingly. There have been instances where New England 

 shells, which had gone to the west coast of America, in the way of 

 exchange, came back again as Pacific shells. 



The second cause is, that shells are regarded as specifically iden- 



c 



