INTRODUCTION. XXIX 



channelled, so that the interior of the shell is also 

 impressed in the same manner for some distance 

 from its edge. When the ridges are once formed 

 on the outside, they remain permanent ; the animal 

 has no power to alter them. It is not so within, for 

 we find that the posterior part is smooth, the fur- 

 rows previously formed by the channelled extremity 

 being worn away, or filled up by the body of the 

 animal. 



