XXXViii INTRODUCTION. [CH. 



the shell. The limpet (Patella vulgata) slightly perforates 

 calcareous rocks by the muscular action of its sucker- 

 like foot or disk, which occupies all the lower part of the 

 body. The shell exactly fits the space thus excavated, 

 so as to prevent the limpet being easily dislodged by a 

 bird or a crab. The common garden-snail excavates 

 hard limestone rocks for the purpose of hibernation; and 

 as its shell is covered with a delicate epidermis, which 

 remains on specimens having just emerged from their 

 winter quarters, it may be safely inferred that the shell 

 is not the instrument of perforation in this case. But 

 with respect to the Teredo or ship- worm (which tunnels 

 through wood), the Pholas (which pierces wood, peat, 

 clay, and chalk), the Gastrochama (which penetrates 

 hard sandstone, chalk, and limestone, as well as old 

 oyster-shells), and the Saxicava (which perforates the 

 hardest calcareous rocks), it is not so easy to form a 

 definite conclusion. This volume would scarcely con- 

 tain all that might be said on the subject. Forbes and 

 Hanley have given an excellent account of the discussion 

 which had taken place up to the time of publishing their 

 work ; and I will content myself with stating briefly the 

 result of their investigation and adding a few remarks 

 suggested by my own inquiries. 



The opinions which had been expressed on this subject 

 when the ' History of the British Mollusca ' was pub- 

 lished, were classed by its authors, when treating of the 

 Pholas, under five conclusions, which are as follows : 



" 1. That the boring Mollusca perforate by means of 

 the rotation of the valves of their shells, which serve as 

 augers. 2. That the holes are made by rasping, effected 

 by siliceous particles studding the substance of certain 

 parts of the animal. 3. That currents of water, set in 

 action by the motion of vibratile cilia, are the agents. 



