I am convinced, then, that the shell forms the _principal_ agent in boring the animal's dwelling, without either acid or flinty particles. The late lamented Professor Forbes held that if this were the case, the rasping points on the surface of the valves would soon be worn down,--an appearance which, he says, is never seen. With all respect for such an eminent name, I must state that he was in error. Not only are the edges at certain times worn, but the rough surface is worn nearly smooth, appearing in certain parts of a white colour, instead of a light drab, as usual. But the reader may ask, if certain parts of the valves are occasionally worn smooth, and the animal works so vigorously, how is it that they are never rasped through? This is a very natural question, and one that I put to myself repeatedly. I have made frequent and careful observations while the animal was actually at work, in order to satisfy myself upon this point, and have always perceived that the particles of softened rock fell from, and on each side of, the large and well-developed _ligament_ that binds the hinge, and extends to the lowest points of the valves. Moreover, this leathery substance always seemed scraped on the surface. I cannot, therefore, but believe that the ligament aids very materially in rubbing off the rock, or at all events, in graduating the pressure of the valves during the process, and that this curious organ, instead of being worn away, may, like the callosity upon a workman's hand, increase in toughness the more labour it is called upon to perform.[13] [13] Mr. Clark says, 'M. Deshayes, in his comment on Pholas, in the last edition of Lanarck, mentions the hinge as scarcely existing, and not being _a veritable ligament_.' How different from the fact; and I will observe, that '_if there is a genus better provided than any other of the bivalves with ligamental appendages, it is Pholas.... The Pholas is iron-bound as to ligament_, which in it is far more powerful in securing the valves, than is the shell of any other group of the acephala, of similar fragility and tenuity!' The reason why so few specimens of the Pholades exhibit a worn shell may be thus explained: As the animal only bores the rock in sufficient degree to admit of its increased bulk of body, it only requires to bore occasionally, and there may be often an interval of many months, during which time nature may have renewed the serrated edge and rough surface of the valves, and thus enabled the creature to renew its wondrous operations. We now come to a consideration of the foot, which, as many writers aver, forms the 'sole terebrating agent.'