3M INTRODUCTION TO CONCHOLOGY. 



holes ; higlier up, their empty shells are alone discoverable j 

 above high water, their perforations only are to be found. 

 Hence it is assumed that all the limestone rocks around 

 Plymouth were under water within the period during which 

 the Saxicavce were employed in their destruction. 



Occasionally the rocks are protected from their aggres- 

 sions by large colonies of Balani, or Sea Acorns, having 

 erected on them their conically-shaped dwellings ; at other 

 times deposits of mud and sand are formed over the rocky 

 base, and hence the operations of boring mollusks neces- 

 sarily cease. 



Great diversity of opinion exists with regard to the 

 means employed by mollusks in forming their perforations : 

 we give a few of those opinions, as expressed in the Geo- 

 logical section of a meeting of the British Association at 

 Plymouth. 



Specimens of perforated rocks brought from Mount Bat- 

 tin were regarded by Dr. Buckland as neither the work of 

 Saxicava nor Pholas, but similar to some discovered at Bou- 

 logne, and ascertained to be occasioned by the common 

 garden-snail. He had observed perforations of the same 

 character at Tenby, and Mr. Topwith spoke of them as fre- 

 quent in Northumberland, on the under side of projecting 



