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Rh The doubts occasionally cast on what are received as established truths in science, though they may seem to unsettle our knowledge, and give a character of vagueness to it, must not be considered as inimical either to its progress or its solidity; not even when, as in the case just cited, they are found to be without foundation. They give rise to new and more careful examinations; to extensive comparisons of species with species, or of fact with fact; to satisfactory inductions of principles from observations; and often to the discovery of laws before unsuspected.

As the family consists but of this single genus, the characters already enumerated need not be repeated. The species are numerous, widely distributed over the globe, scarcely any sea being destitute of some, with the exception of the Arctic Regions, where none have been observed by voyagers. As usual, the largest species are found in the seas of the tropics. Deshayes in his Tables enumerates 104 living species, and ten fossil; several others have been added by subsequent naturalists, but as the genus is peculiarly liable to variation in the form, colour, and surface of the shell, it is very likely that many of these described species are merely varieties. The animals of this genus have the power of wearing away, or of absorbing the surface of other shells, or of the rocks to which they adhere, and of thus forming sunken pits or depressions on them. The Patella cochlear of the Cape of Good Hope, is often found attached to a large species of the same