Until the middle of the 18th century, naturalists who studied the marine fauna and flora relied for their materials on shore collection and the examination of the catches of fishing boats. Their knowledge of creatures living below the level of low spring tides was thus gained only from specimens cast up in storms, or caught by fishing gear designed for the capture of certain edible species only. The first effort made to free marine biology from these limitations was the use of the dredge, which was built much on the plan of the oyster dredge. [Illustration: FIG. 13.--Otho Frederick Müller's Dredge (1770).] _The Oyster Dredge._--At first naturalists made use of the ordinary oyster dredge, which is constructed as follows. The frame is an iron triangle, the sides being the round iron "arms" of the dredge, the base a flat bar called the shere or lip, which is sloped a little, not perpendicular to the plane of the triangle; an iron bar parallel to the base joins the arms. The net is fastened to the parallel bars and the portion of the arms between them, and consists of two parts: that attached to the shere is of round iron rings linked together by smaller ones of wire lashings, that attached to the upper bar is of ordinary network. Where these two portions of the bag meet a wooden beam is fastened. In use the frame is towed forward by its apex: the shere passes below oysters, &c., which pass back on to the iron netting. The length of each side of the triangular frame is about 6 ft., the width of the shere 3 in. and the height of the mouth just under a foot. The rings vary in size, but are usually some 2½ in. in diameter. The weight is about 60 lb. This dredge was soon abandoned: its weight was prohibitive for small boats, from which the naturalist usually worked, its wide rings allowed precious specimens to fall through, and its shallow net favoured the washing out of light objects on hauling through the moving water of the surface. Moreover, it sometimes fell on its back and was then useless, although when the apex or towing point was weighted no great skill is needed to avoid this.