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The means by wliich these animals penetrate dense calcareous 

 substances, has been discussed by many able writers, without a 

 satisfactory solution of the problem. Some have Supposed that 

 the operation is eifected by the friction of the valves of the 

 shell ; but the valves of some species are very thin and not so 

 dense as the substance they penetrate, and never exhibit any 

 abrasion of their attenuated edges. Others contend that a pecu- 

 liar acid or solvent must be secreted by some appropriate organ, 

 which dissolves the rock by a chemical action ; but neither 

 anatomy nor chemistry have exhibited proofs in support of this 

 opinion, and in this state of uncertainty we are still left to con- 

 jecture and analogy. We know that the power of penetrating 

 calcareous substances, as well as wood and extremely dense 

 earth, is not confined to animals of this family, but that many 

 others bore through shells to devour the inhabitant, with too 

 small a hole to admit any part of their own shell, and numerous 

 other species, as their whirls revolve in the growth, remove the 

 asperities of the preceding volution as the aperture approaches 

 them. This efibct is observable in almost all rough univalve shells ; 

 some, indeed, cover their slight inequalities with the calcareous 

 deposite of the labium, but whenever the inequality is prominent, 

 it is sui-e to be removed at the aperture, and it would seem that 

 the operation may possibly be, in some instances at least, effected 

 by the constant action of the soft parts of the animal, or by the 

 agency of absorbents acting on the ultimate particles. This opera- 

 tion is by no means extraordinary, as every anatomist is aware that 

 the bony portions of the animal frame are universally modified by 

 the action of the softer parts. In many of the Annclidcs we find 

 animals of a very soft, almost gelatinous structure, penetrating the 

 hardest calcareous rocks, and into the substance of the thick valves 

 of many shells. These analogies lead us to the conclusion that 

 the Lithophaga excavate a lodgement in solid substances not by 

 the friction or boring of their shells, but by the operation of their 

 soft parts upon them, and not, as a distinguished naturalist has 

 recently supposed, exclusively by maceration of their animal mu- 

 cus. There are, however, some facts which seem to indicate the 

 presence of a solvent. Mr. Osier has a specimen of a bard cal- 

 careous rock in which small masses of silex remain in relief on 

 the sides of excavations formed by Saxicava rugosa and Venerupu 



