INTRODUCTION. *> 



most imperceptible variations of structure, so eminently 

 characteristic of the transitions from one genus to ano- 

 ther, and so necessary to enable the naturalist with 

 greater confidence to determine in which the object 

 under his examination should properly be classed. 



The inevitable consequence of this melancholy pri- 

 vation, has been some anomalies and inaccuracies: they 

 are however of less importance, and fewer in number, 

 than could have been expected in a work so voluminous, 

 continued under such unfavorable and discouraging 

 circumstances. 



La Marck, in that part of his work descriptive of 

 the Molluscse having testaceous coverings, has been 

 entirely guided by the organization of the animal, in 

 forming an arrangement of the genera of their shells. 

 It being a well established fact, that the animal con- 

 structs the shell by which it is wholly covered, or 

 only partially protected, on the model of its naked 

 body, by successive superincumbent strata of testa- 

 ceous matter, the first being merely a thin, fragile, 

 or viscous Substance, becoming of greater thickness 

 and solidity by each additional deposit, until it ul- 



