35 



GENERIC SYSTEM. 



We come now to the arrangement of Shells, after 

 the method of Linnaeus ; and if we bear in mind 

 the professed foundation of that arrangement, we 

 shall find but little cause to complain, either of per- 

 plexity in the general scheme, or of want of pre- 

 cision in the several descriptive parts. It is upon 

 external characters, upon those of the testaceous 

 covering, and not upon the genus or species of the 

 worm, that we are to erect our system ; because 

 the former are the most obvious, and the least 

 liable to misconception. 



Premising, then, that by the word Valve is 

 meant any single piece of calcareous substance, let 

 the form be what it may, which serves as a habi- 

 tation, or protection, either partially or entirely, 

 to a Molluscous animal, we shall understand the 

 natural division of all shells into Multivalves, those 



