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The means by which 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 effected 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 effect 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 sure 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 Annelides 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 hard cal- 
careous rock in which small masses of silex remain in relief on 
the sides of excavations formed by Saxicava rugosa and Yeneruip'h 
