In France oyster dredging is conducted by fleets of thirty or forty boats, each carrying four or five men. At a fixed hour, and under the surveillance of a coastguard in a pinnace bearing the national flag, the flotilla commences the fishing. In the estuary of the Thames the practice is much the same, although no official surveillance is observed. Each bark is provided with four or five dredges, resembling in shape a common clasp purse. It is formed of network, with a strong iron frame, as represented in Fig. 172, the iron frame serving the double purpose of acting as a sucker, and keeping the mouth open, while giving it a proper pressure as it travels over the oyster-beds. When the boat is over the oyster scarp, the dredge is let down, and no more attractive sight exists than that presented by the well-appointed Whitstable boats on one side of the estuary, or the Colne boats on the other, as they wear and tack over the oyster-beds, bearing up from time to time to haul in the dredge, and empty its contents into the hold. The tension of the rope is the signal for hauling in, and very heterogeneous are the contents--sea-weeds, star-fishes, lobsters, crabs, actinia, and stones. In this manner the common oyster fields on both sides of the Channel were ploughed up by the oyster dredger pretty much as the ploughman on shore turns up a field. The consequence was that, twenty years ago, the French beds were totally exhausted, and France had to look to foreign countries for its oyster. Oyster farms which had employed fourteen hundred men and two hundred boats were reduced to two hundred men and twenty boats. Similar results from over-dredging would have followed, no doubt, on this side the Channel had the mollusc not been protected by the company and private proprietors who held the oyster-beds in the large estuaries. This state of things in France led to some important discoveries in the science of oyster culture, which have produced important changes there. [Illustration: Plate XII.--Dredging for Oysters.] [Illustration: Fig. 172. Dredge employed in Oyster fisheries.]