4 IXTRODUCTION". 



abide, a sufficient number of live and perfect spe- 

 cimens. 



Of those wliich are more readily attainable, 

 there arc parts and habitudes very difficult to be 

 accounted for, which yet may constitute an essen- 

 tial 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 consc(iucntly the 

 diffusion of the science, must be vcrv 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 likelv to re- 

 main so, it becomes necessary to resign all hopes 

 of a zoological arr:u)gement similar to that of the 

 other classes of the kingdom of nature ; wo have 

 onlv therefore to take care, as far as our knowledge 

 does extend, not to admit any incongruities into 

 the system which we arc compelled to adopt, but 

 to frame it as much in consistency as possible with 

 the natural alliance both of tiie worms and shells. 

 If errors have crept in, arising from ignorance or 



