[T]he testaceous tribe receive also their growth from a like internal circulation. If it be urged, that this does not exclude the possibility of a like shell being produced by the passage of a fluid through the pores of the circumjacent body, whether of earth, stone, or water; I answer, that it is not within the usual economy of nature, to use two processes for one species of production. While I withhold my assent, however, from this hypothesis, I must deny it to every other I have ever seen, by which their authors pretend to account for the origin of shells in high places.

Thomas Jefferson