INTRODUCTION. XXV11 



Of the Tubercles, Spines, fyc. which are found on 



Shells. 



It is sufficiently evident, that any pliable sub- 

 stance will assume the form of the body on which 

 it is moulded, and that, consequently, if the animal 

 inhabitant of a shell has any. muscular inequalities, 

 the same will be observable in its exterior con- 

 formation. This point being allowed, there is no 

 longer any difficulty in accounting for the warts, 

 and spiny processes, on the backs of shells, which 

 may be considered as so many testaceous incrusta- 

 tions, deposited by corresponding fleshy tubercles. 

 These excrescences may often be observed in the 

 first turn of a spiral shell, and they will be found, 

 more or less, in every succeeding spire which the 

 animal adds to its habitation. This is particularly 

 evident in the Mnrex Mancinella, which is frequently 

 so covered with tubercles, as to be much disfigured 

 in its shape. The long spines, which appear on 

 some species of the above genus, have the same 

 origin as the shapeless knobs just mentioned. They 

 have their interior moulds upon which they are 

 formed, and are either hollow, partly solid, or per- 

 fectly closed, in proportion as the animal excres- 



