We all know that the hole which each young Pholas makes, when first he takes possession of his rocky home, is extremely minute,--not larger than a small pin's head; now, it stands to reason, that if the shell was the only terebrating agent, the opening of the cavity in question would always remain of the same size, or, perhaps, on account of the action of the water, a slight degree larger than its original dimensions. Such, however, is not the case. Here is a fragment of rock exhibiting several Pholas holes. The aperture of one of these, which I measure, is nearly half-an-inch in diameter, while in juxtaposition with it is situated another cavity, measuring across the entrance less than the eighth part of an inch. The reader will at once perceive, if the foot and shell were the sole augurs, that as the animal descended deeper into the rock, the siphonal tube, as it enlarged in proportion to other parts of the animal, would have to be drawn out to an extremely fine point to fit the opening of the tunnel. But as this is not the state of matters, the conclusion forces itself upon us, that that portion of the orifice situated above the shell of the animal must be enlarged by the constant extension and retraction of the siphons, aided by currents of water acting on the interior surface of the cavity. This same theory will also serve to explain how it is that all Pholades situated at the same depth in the rock, are not all of a uniform size. I have frequently seen a piece of rock exhibit the peculiarity of two burrows of vastly different proportions as regards breadth, being precisely the same depth from the surface of the stone. This appears to me equally wonderful and puzzling at first sight, as the 'boring' question. What age is attained by any species of the rock-borers before they arrive at full growth, there are no means of knowing. This point, like several others in the history of these animals, still remains a mystery, nor is it likely soon to be cleared up. The largest specimen of _P. crispata_ that I have seen is at present in my possession. Each valve measures three and a half inches in length, by two inches in breadth. Some foreign specimens of this species, and especially of _P. dactylus_, are, however, frequently found of much larger dimensions. On no occasion have I ever examined any Pholas excavation that had lost its conical shape, a fact that seems to prove that the successive stages of the boring operation must have taken place solely in consequence of the animal not having reached its adult form.[15] For had the shell attained its full development, and its owner continued to labour, and rasp away the rock, the sides of the cavity at its base would necessarily present a parallel appearance--a phenomenon which is never witnessed. [15] The above remark holds good, even although (_as is frequently the case_) the animal wilfully deviates from the straight path, and bores its tunnel in a curved form.