The neighbourhood of Swansea offers remarkable facilities for observing the habits of the burrowing marine animals. Many Spatangi, innumerable Annelides, and a variety of bivalves are found on its extensive sandy shores; considerable beds of decayed wood are inhabited by Pholas Candida; and the rocks at the western extremity of the bay abound with Lithophagi. My chief object in this paper, is to explain the mechanism by which the boring and burrowing shell fish form their habitations; but as there are facts connected with the burrowing of other marine animals which are yet but imperfectly understood, I shall first advert briefly to the latter.