INTRODUCTION. 1X 
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 they 
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- 
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