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Rh and fastens it securely. Having accomplished this the foot is retracted; and the thread, of course, being drawn out of the furrow where it was secreted, is added to the bundle of byssus previously existing, all of which owed its origin to a similar process."

Whoever has attempted to wrench up a Mussel from one of those shallow rock-pools, in which they lie as closely packed as paving stones, will have had proof of the great strength of these threads, no small violence being required to detach one. But there is an example on record, where the strength of the threads has been turned to such account as to give this Mollusk a second claim to be included in the list of such species as are beneficial to man.—"At the town of Bideford, in Devonshire, there is a long bridge of twenty-four arches across the Torridge river, near its junction with the Taw. At this bridge the tide flows so rapidly that it cannot be kept in repair by mortar. The Corporation, therefore, keep boats in employ to bring mussels to it, and the interstices of the bridge are filled by hand with these mussels. It is supported from being driven away by the tide entirely by the strong threads these mussels fix to the stonework; and by an act, or grant, it is a crime liable to transportation for any person to remove these mussels, unless in the presence and by the consent of the corporative trustees."

There are bivalve shells allied to the mussel, called Pinna, usually of very large size, but of thin and delicate structure. The threads spun by these are long, fine, glossy, and produced in great abundance; they are capable of being twisted like silk,