PREFACE. VU. 



Conchology may therefore form a distinct class, seperate from 

 the MoUusca, reducible to five orders, from their own permanent 

 and unalterable characters. 



1. MoNOTHYEA. Testa univalvis, libera. 



Shell univalve, and free. 



2. DiTHYEA. Testa bivalvis, accessoriis difformibus r arias 



instructa. 

 Shell bivalve, rarely furnished with diiferently 

 formed accessorial valves. 



3. PoLYTHYEA. Testa multivalvis. 



Shell with several similar valves. 



4. MoNOTHALAMiA. Testa tubularis, monothalamia, fixa. 



Shell tubular, of one cell, and fixed. 



5. PoLYTHALAMiA. Testa polytholamia, libera seu fixa. 



Shell of several chambers, free or fixed. 



The subject of our present Essay is the second order, compre- 

 hending the bivalve shells of the British islands, reducible into 

 divisions by the absence or position of the ligament of union, and 

 into families or genera by the absence, number, or peculiarities of 

 the teeth. 



