4598 Mollusks. 



hypothesis in connexion with the shell as the perforating instrument, for though it may 

 explain the burrows made by the Pholas, it is utterly inapplicable to the case of other 

 stone-borers, some of which, though not cylindrical, occupy excavations of the same 

 shape as their shell, and which excavations consequently could not be scooped out by 

 a rotatory motion of this ovgan, no rotation being possible ; while other kinds, although 

 like the Pholas of a cylindrical shape, instead of having the shell rough and file-like, 

 have it covered with a delicate membrane, of such a nature that any friction of the 

 shell against a hard substance would inevitably effect its destruction ; yet this mem- 

 brane is found present and uninjured on shells of this particular kind at all periods of 

 their growth. The next hypothesis reviewed was the chemical one — that which sup- 

 poses stone-boring shell-fish to secrete an acid or a peculiar solvent capable of dissolving 

 all the various substances into which these creatures burrow, not even excepting wax. 

 Mr. Charlesworth enumerated as difficulties in the way of accepting the chemical hy- 

 pothesis, that, after repeated and most carefully conducted experiments, chemists had 

 failed in detecting the smallest trace of an acid secretion in the skin or other soft 

 parts of stone-boring shell -fish ; that if any such assumed acid or solvent do really 

 exist, its properties must be of a most novel and extraordinary kind, from the variety 

 in the chemical constitution of the substances upon which it acts,— wax, wood, lime- 

 stone, &c. &c. ; that it would, upon this hypothesis, be difficult to understand how it 

 is that the solvent in question does not act upon the creature's own shell, as well as 

 upon the walls of the burrow or cell in which that shell is lodged ; and lastly, as the 

 work is carried on under water, it would be necessary to assume some provision for the 

 application of the solvent without the accident of its dilution by the surrounding 

 fluid, a provision which would hardly appear possible in some cases, such, for instance, 

 as the excavations made by Gastrochaena and Lithodomus, in the highly porous sub- 

 stance of the common brain coral. In conclusion, the attention of the meeting was 

 directed to the views and elaborate researches of Mr. A. Hancock, of Newcastle, 

 bearing upon the elucidation of this long-agitated problem in Natural History. Mr. 

 Hancock states that, upon submitting to microscopic examination that portion of the 

 boring shell-fish designated by comparative anatomists as the "foot," he found it 

 studded with minute crystalline bodies, upon which acids had little or no effect : hence 

 he assumes these bodies to be silicious, and, detecting their presence in other organs 

 of the animal besides the foot, he founds upon this and other considerations most ably 

 treated of in his published memoir upon the subject an hypothesis that the burrows of 

 shell-fish are formed by a mechanical process, but that the soft parts of the animal 

 i.rmed with these silicious studs or points are the boring instrument, not the shell. In 

 reference to this last hypothesis, Mr. Charlesworth stated that he thought it far more 

 philosophical than any of the theories which Mr. Hancock's predecessors in this field 

 of research had advanced. He still, however, saw difficulties in the way of at once 

 adopting it. Other naturalists, however, would doubtless follow the line of investi- 

 gation pointed out by Mr. Hancock, and independent observations would, ere long, be 

 forthcoming in confirmation or refutation of this gentleman's views. A highly inte- 

 resting series of specimens of stones, wood and coral, showing the boring shell-fish in 

 their burrows, were exhibited by Mr. Charlesworth in illustration of his remarks. 



