346 INTRODUCTION TO CONCHOLOGY. 



mal iu breathing might have the power of softening his 

 rocky dwelling-place. Dr. Buckland observed that if the 

 perforations were owing to currents^ the greater force would 

 be exerted at the aperture^ and consequently render that 

 the larger part^ while the interior of perforations made by 

 PJiolades at Lyme Regis^ were marked by parallel circular 

 strise, owing to the mechanical action of the shell, which 

 rasps away the rock, and increases the size of the cavity 

 in exact proportion with its growth. The perforations at- 

 tributed to land-mollusks were entirely chemical. Perfora- 

 tions in limestone rocks brought from Barnstaple to the 

 Cambridge Museum, were mentioned by Professor Henslow 

 as being hollowed by the chemical action of muriate of lime 

 having converted some parts into carbonate of soda. Whe- 

 ther such perforations are accounted for by the action of 

 carbonic acid, or the agency of currents, certain it is that 

 these mollusks estabhsh themselves not only in limestone, 

 but trap-rocks, and those of the Old Eed Sandstone. 



The shells of the Petricola, w^hich pertain to the same 

 family, assume also a similar irregularity of growth, from 

 the habit of burrowing : they may be readily distinguished 

 from those of Saxicava by the teeth being more fully deve- 

 loped, and by having a large sinus in the pallial impression. 



