INTRODUCTION. 



CLASSIFICATION. 



The classification of shells, that is, their systematic arrange- 

 ment into classes, orders, families, genera and species, cannot 

 be made to depend entirely upon the characters observable in 

 them, viewed by themselves; for this reason, that many 

 similarly formed sheUs belong to animals perfectly distinct, 

 and that many molluscous animals are found to agree 

 with each other in every respect but in the form of their 

 testaceous support. There are, however, many important dis- 

 tinctions to be observed in the shells themselves, leading to the 

 estabhshment of many of those very divisions, which would 

 afterwards be confirmed by an examination of the soft parts. 

 It is desirable to attend, as far as means and opportunity will 

 allow, to all the points of difference, in order to form, or even 

 to appreciate, a generic or wider distinction. It will there- 

 fore be our endeavour to explain the general principles upon 

 which those distinctions are formed, and the manner in which 

 they are appHed and expressed in detail by scientific writers. 



NUMBER OF PIECES, OR INDEPENDENTLY 

 FORMED PARTS. 



The first, most simple and obvious division of shells, is that 

 which results from the number of separate pieces composing 

 them. Hence the distinction implied by the terms univalve, 

 or consisting of a single piece ; bivalve, or composed of two 

 pieces; and multivalve, or composed of more than two. 

 For an example of univalve, take a common whelk; for a 

 bivalve, take a muscle or a scallop ; and for a multivalve, the 

 chiton. 



But although this arrangement may appear at first sight 



