256 AUTHOR'S REMARKS ON THE 



works so vigorously, how is it that they are never 

 rasped through ? This is a very natural question, 

 and one that I put to myself repeatedly. 



I have made frequent and careful observations 

 while the animal was actually at work, in order to 

 satisfy myself upon this point, and have always 

 perceived that the particles of softened rock fell from, 

 and on each side of, the large and well-developed 

 ligament that binds the hinge, and extends to the 

 lowest points of the valves. Moreover, this leathery 

 substance always seemed scraped on the surface. I 

 cannot, therefore, but believe that the ligament aids 

 very ..materially in rubbing off the rock, or at all 

 events, in graduating the pressure of the valves 

 during the process, and that this curious organ, 

 instead of being worn away, may, like the callosity 

 upon a workman's hand, increase in toughness the 

 more labour it is called upon to perform. 1 



The reason why so few specimens of the Pholades 

 exhibit a worn shell may be thus explained: As the 

 animal only bores the rock in sufficient degree to 

 admit of its increased bulk of body, it only requires 

 to bore occasionally, and there may be often an in- 

 terval of many months, during which time nature 



1 Mr. Clark says, ' M. Deshayes, in his comment on Pholas, in the last edition oi 

 Lanarck, mentions the hinge as scarcely existing, and not being a veritable liga- 

 ment.' How different from the fact ; and I will observe, that l if there is a genus 

 better provided than any other of the bivalves with ligamental appendages, it is Pholas. 

 ..... The Pholas is iron-bound as to ligament, which in it is far more powerful in 

 securing the valves, than is the shell of any other group of the acephala, of 

 similar fragility and tenuity 1 ' 



