and on the ranoval of portions of thel/r Shells, 231 



shell. The edges of small cavities cut through remain also per- 

 fectly sharp. 



' It is not uncommon to find at the bottom of the burrows of 

 Saxicava a fine sediment, apparently arising from the scourings 

 of the excavation. If this be carefully removed with the point of 

 a pen and immersed in acetic acid, effervescence ensues. A similar 

 sediment on the back of the shell of Gastrochana also effer- 

 vesced when treated in the same way. And if a little of the same 

 acid be applied to the anterior portion of the animal of Samcava, 

 — that portion where Mr. Osier rightly considers the excavating 

 instrument is situated, — the same result occurs ; a convincing 

 proof that calcareous matter was adhering to it, and strongly in 

 support of the opinion that the boring instrument is not armed 

 with an acid solvent. These experiments seem much in favour 

 of mechanical action. 



And after all, how is a solvent to be applied with effect under 

 water ? One way suggests itself. The surface of an adhesive 

 disc firmly attached to the part to be eroded might secrete, and 

 at once apply the necessaiy fluid : but even in this case there is 

 a difficulty ; for in exact proportion to the quantity secreted, the 

 attachment of the disc would be diminishwi, and the surround- 

 ing element coming in contact with the solvent, it of course must 

 be diluted. May the solvent be of an oily nature, or of such a 

 character as not to mingle with water ? If so, how is it to act on 

 a substance saturated with water ? But with Saxicavu the diffi- 

 culty is increased, for it is not furnished with such an adhesive 

 disc : in this respect it differs from PJiolas and Teredo, 



The burrows of Saxicavu on the Northumberland and Durham 

 coasts occasionally pass through the soft, porous parts of the 

 magnesian limestone, which are completely saturated with water. 

 It might, therefore, be supposed that any solvent secretion would 

 be so diluted as to be rendered impotent. And it is common for 

 Lithodomus and several other borers to bury themselves in corals, 

 some of which, on account of their open texture, must contain 

 large quantities of water. Mr. Fryer of Whitley possesses a spe- 

 cimen of coral thus excavated, which has the cells so large that 

 the sides of the burrows resemble a section of a bundle of small 

 crow-quills. In this case it is almost impossible to conceive how 

 any solvent could act on the thin laminae that divide the cells, 

 which, of course, would be filled with water ready at every point 

 to mingle with and to destroy the corrosive effect of any fluid 

 secreted bv the animal. 



Turton entertains the opinion that the Pholades penetrate by 

 the aid of phosphoric acid. He adopted this notion from having 

 investigated the habits of P. dactylus, P. parva and P. Candida 

 on the Devonshire coast, where, according to his account. 



