4 INTRODUCTION. 



recesses in which many, if not the majority, abide, 

 a sufficient number of live and perfect specimens. 



Of those which are more readily attainable, 

 there are parts and habitudes very difficult to be 

 accounted for, which yet may constitute an es- 

 sential difference in the animal. Were we indeed 

 able to obtain the inmates of every known shell, 

 and submit them to an accurate examination, still 

 the pleasure of arrangement, and consequently the 

 diffusion of the science, must be very partial, as it 

 would necessarily be confined to skilful anatomists 

 and profound philosophers. 



Our knowledge of the animals being so ex- 

 tremely limited at present, and being likely to re- 

 main so, it becomes necessary to resign all hopes 

 of a zoological arrangement similar to that of the 

 other classes of the kingdom of nature ; we have 

 only therefore to take care, as far as our knowledge 

 does extend, not to admit any incongruities into 

 the system which we are compelled to adopt, but to 

 frame it as much in consistency as possible with 

 the natural alliance both of the worms and shells. 

 If errors have crept in, arising from ignorance or 



