GENERIC SYSTEM. 
W E 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 
