50 



GENERIC SYSTEM. 



We come now to the arrangement of Shells, after 

 the method of Linnaeus 3 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 Falve is 

 meant any single piece of calcareous substance, 

 let the form be what it may, which serves as a 

 habitation, or protection, either partially or en- 

 tirely, to a Molluscous animal, we shall understand 

 the natural division of all shells into Multivalves, 

 those composed of more than two pieces 5 Bivalves, 

 of two distinct parts 5 and Univalves, of one en- 

 tire formation. 



