xl INTRODUCTION. [CH. 



cave end of the tube. The shell of Saxicava is coated 

 with a dehcate epidermis, which would unquestionably 

 be scraped away in perforating the solid limestone rock, 

 if the theory advocated by Forbes and Hanley is appli- 

 cable to this case. And, to take the case of other 

 marine animals which excavate rocks and shells for the 

 same purpose as the Mollusca, we know that the boring 

 Annelids or sea-worms have no hard substance in their 

 composition. For these reasons, I do not think that 

 the mechanical theory (viz. that the shell is the sole 

 instrument of perforation) has been established. 



Since the publication of Forbes and Hanley^'s work, the 

 controversy has been continued with unabated ardour ; 

 and to the list of naturalists who have taken a part in 

 it, other names may be added as supporters of the under- 

 mentioned theories. 



Mechanical. Mr. Robertson and M. Fischer. 



Chemical. Dr. Mantell, M. Thorent, and Mr. Reeve. 



M. Cailliaud now contends that both methods of per- 

 foration are adopted by the same kind of mollusk ac- 

 cording to the material acted upon ; and M. Bouchard- 

 Chantereaux, who was at first in favour of the mechani- 

 cal theory, is now strongly of opinion that a corrosive 

 secretion of the animal is the agent of perforation. 



But there is another point of view in which the ques- 

 tion may be considered, and which does not appear to 

 have received any attention, although in my humble 

 judgment fully deserving it. 



Nearly 130 years ago, a very learned but eccentric 

 Dutch philosopher, named Sellius, wrote and published, 

 for the benefit of his country, an elaborate monograph on 

 the Teredo. In this remarkable production he discussed 

 at great length, and in the most exhaustive style, all 

 the various theories which had been propounded up to 



