INTRODUCTION. 9 



be made to depend entirely upon the characters observable in 

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

 larly formed shells form the habitations of 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 

 establishment of many of those very divisions, which would 

 afterwards be confirmed by an examination of the soft parts. 

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

 allow, to all the points of difference, both in the shell and in 

 the animal, in order to form, and in some instances even to 

 appreciate, a generic or larger distinction. It will therefore be 

 our endeavour to explain the general principles upon which 

 those distinctions are formed, and the manner in which they 

 are applied 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 

 barnacle, or balanus, found adhering to the common oyster. 



But although this arrangement may appear at first sight 

 perfectly easy and plain, some explanation will be necessary in 

 order to guard the student against understanding the above ex- 

 pressions in their strictest sense, without qualification. Thus the 



