I.] INTRODUCTION. xiU 



ought to satisfy any naturalist, who is inclined to con- 

 sider the question in an unprejudiced spirit, that the 

 body or soft parts of the mollusk, taken without re- 

 ference to the shell, offers an extremely sHght and 

 variable criterion of specific diff'erence. The shell of 

 itself generally enables the conchologist to distinguish 

 one species from another, without regard to the soft 

 parts ; and as the latter are seldom observable, the con- 

 venience of such a mode of distinction is obvious. It 

 would be rather difficult for a malacologist to describe 

 any particular kind of testaceous moUusk without no- 

 ticing the shell ; and for the same reason a crab or sea- 

 egg would not be easily recognized by the description, 

 if all mention of the carapace or test were omitted. The 

 shell of the moUusk may be in some respects considered 

 as a pseudo-skeleton, serving not only to protect the 

 soft and tender body, but also to keep the whole frame 

 together, like the true skeleton of any vertebrate animal. 

 Tliere is, besides, an intimate connexion between the 

 shell and the tissues of the body, which is only dissolved 

 by death or violence. The shell is (to use the words of 

 Mr. Searles Wood) "part and parcel of the animal itself." 

 I am aware that this opinion has been controverted by 

 high authority, and especially by Dr. Gray, whose valu- 

 able contributions to the science of zoology, in many of 

 its branches, are familiar to all. He, at one time, dis- 

 turbed the minds of geologists as well as students not a 

 little, by a statement that some shells which were per- 

 fectly alike were inhabited by animals so extremely 

 dissimilar as to be referable to very different orders of 

 MoUusca^. This statement, however, he afterwards quali- 

 fied to a considerable extent by admitting that, " in the 

 distinction of the larger and smaller groups of Mollusca, 

 * Phil. Trans. 1834, p. 302. 



