Page:Natural History, Mollusca.djvu/310

 such particles, though inclined to regard such a view with favour, as in this case the surface of the shell does not seem devised for rasping, as is that of the shells of the Pholadidæ''

(Borers.)

The members of this family have a very close affinity with those of the last, both in structure and habit, and have indeed been often united with them by zoologists. They possess peculiarities, however, which make their separation more natural. The genera differ among themselves more in outward appearance than in organization; they have the body usually lengthened in various degrees, and produced behind into very long siphons, which are united through their length. The mantle is closed except in front, where the foot protrudes; this organ is club-shaped, and has the extremity obliquely flattened, so as to form a broad disk; it is sometimes largely developed, at others minute. The valves are equal, but very one-sided, always gaping at both extremities; the hinge is without teeth, and there is no proper ligament; each valve is furnished with a curved calcareous process beneath its summit, and at the back of the shell there are generally two accessory plates resembling valves.

The mollusks of this group are as indomitable borers as the Gastrochænadæ, but they less exclusively confine themselves to hard substances.