230 Mr. A. Hancock on the Boring of the Mollusca into Rocks, 



shell, shale, shaly sandstone, siliceous grit and wood, it appears 

 incapable of injuring the shell of the animal that secretes it. 



It has been urged by those naturalists who advocate the ex- 

 istence of such an unknown solvent, that it may be applied in 

 such a way as not to touch the calcareous covering of the animal, 

 and that vitality may resist ordinary chemical action. It is dif- 

 ficult to understand how this can be ; and it is quite certain that 

 the shell of the living Saxicava rugosa cannot retard the pene- 

 trating power of another individual of its own species. Mr. Osier 

 states f that when the holes communicate, it is common to meet 

 with one animal which has attached its byssus to another, 

 and that in such case the second is always acted on by the as- 

 sailant ; and also that it frequently happens under such circum- 

 stances for the one to penetrate completely through the shell of 

 the other." And this gentleman supposes, that when an animal 

 is thus wounded, a " firm yellow substance " which it secretes is 

 sufficient to resist the further progress of the intruder. It seems 

 strange that if this substance, which is of a coriaceous or horny 

 appearance, does really retard the progress of the animal, the 

 epidermis should not have done so in the first instance. I have 

 seen nothing in confirmation of Mr. Osier's supposition, though 

 I have frequently observed what he describes ; but can readily 

 understand that the infliction must cease on the destruction of 

 the shell to which the byssus, as before stated, was attached : 

 this will be evident when we come to consider the animal, and 

 the mode by which I think it penetrates. 



Mr. Osier further states, that " the cuticle which had necessa- 

 rily been exposed to the same agent remains uninjured and over- 

 hanging the breach." This no doubt occasionally happens, but 

 in all the cases that I have seen, the epidermis has been entirely 

 destroyed. I cannot however conceive that this favours the sol- 

 vent theory, as Mr. Osier supposes : on the contrary, it seems to 

 prove that friction must have been used, for how could a solvent 

 that is unable to reduce the epidermis ever reach the shell be- 

 neath it ? This can only be explained by supposing that it had 

 been torn by friction, and pushed aside so as to expose the sur- 

 face of the shell. 



When the burrows communicate, which they frequently do, 

 the edges of the passage of communication are always very sharp, 

 so much so, indeed, as to favour strongly the theory of friction ; 

 for had a solvent been used, the probability is that those edges 

 must have been more or less rounded. It is the same when the 

 burrows of Teredo run into each other, which sometimes happens. 

 And when the shell of Saxicava has been completely cut through 

 by an assailant, the plane of the section is quite flat, and forms 

 sharp 'land perfect angles with the inner and outer surfaces of the 



