4 
INTRODUCTION. 
abide, a sufficient number of live and perfect spe- 
cimens. 
Of those which are more readily attainable, 
there are 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 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 
remain 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 
