PREFACE. Vll 



Cuvier, will be found to coincide with singular " semblable 

 coherence." 



Conchology may therefore form a distinct class, separate from 

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

 and unalterable characters. 



1. MoNOTHYRA. Testd univulvis, libera. 



Shell univalve, and free. 



2. DiTHYRA. Testa bivalvis^ accessoriis difformibus rarius 



instruct a. 

 Shell bivalve, rarely furnished with difierently 

 formed accessorial valves. 



3. PoLYTHYRA. Testa multivalvis. 



Shell with several similar valves. 



4. MoNOTHALAMiA. Tssta tubularis, monothalamia, fixa. 



Shell tubular, of one cell, and fixed. 



5. PoLYTHALAMiA. Testa polythalamia, libera sen Jixa. 



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. 



