To Robert Ball   8 June [1851]

My dear Sir

I had intended thanking you sooner for your kindness in taking the trouble to send me the cast & very curious account of the “mines” in the turtle-shell.¹

I am more than ever confounded, on the whole subject, for I sh^d. have said that nothing was more certain than that a cirripede could not have lived in such a chamber as you describe, not openly connected with the sea-water.— You can hardly think more lightly about my notions of the burying of Tubicinella Coronula &c, than I do myself: I see, however, I omitted stupidly one important point in my notion, viz that the attachment of the cirripede injures the corium or true skin beneath the epidermis on which it is fixed, by means of a peculiar substance, which I call cement, which every cirripede secretes at its basis— When attached, the growth of the shell must produce pressure, & would not absorption follow, as in such case when a curved tusk of a Boar or horn of cow has penetrated the bones of the head?² But I can only repeat that I am quite confounded on the subject.—

I have not yet come to the main embedded genera, & will certainly attend to the subject.³ I know now enough to feel sure that they do not burrow mechanically.—

With many thanks | Pray believe me | My dear Sir | Yours sincerely | C. Darwin