The “Periwinkles” (_Littorinidæ_) are found almost on every known shore; they feed upon all kinds of marine vegetation. Some species are met with at low-water mark, others on rocks almost beyond the reach of the sea, and some have been discovered inland nearly half a mile away from the shore. It is calculated that 1900 tons of the “Common Periwinkle” (_Littorina littorea_), of the value of £15,000, are annually consumed in London alone. [Sidenote: Cases 31–32.] The family of _Calyptræidæ_ includes the “Slipper-Limpets” (_Crepidula_) and the “Cup-and-saucer Limpets” (_Crucibulum_). Although furnished with a foot, they rarely crawl about, but remain attached to rocks, stones, or other shells, sometimes forming a shelly plate under the foot by which they become fixed to the spot where they have taken up their abode. [Sidenote: Cases 32–35.] The “Cowry-shells” (_Cypræidæ_) are remarkable for their varied markings and splendid polish, which is produced and preserved by two flaps of the mantle, one on each side, which fold over the back, a line down the centre of which usually marks where the flaps meet. The animals are even more brilliantly coloured than the shells. They have no operculum, but a large foot, which they can withdraw entirely within their shell, although the aperture is usually very narrow. Cowries, as is well known, are sold as ornaments; and a small yellow species, “the money-cowry” (_C. moneta_), which is very common in the Indian and Pacific Oceans, passes current as coin among the negro tribes of certain parts of Africa. The specimen of _Cypræa leucodon_ figured on p. 16 is extremely valuable and supposed to be the only one hitherto discovered. The “orange cowry” (_Cypræa aurora_) is worn by chiefs in the Friendly Islands, and is considered the highest order of dignity. Only one small species, _Trivia europæa_, is found on the British coast, and about 100 fossil forms have been discovered in the Chalk. [Illustration: Fig. 9. The Tiger Cowry (_Cypræa tigris_). (From the Indo-Pacific Ocean.) _a_, the shell; _b_, the mantle; _c_, foot; _d_, siphon; _e_, proboscis; _f_, tentacles; _g_, eyes. ] [Sidenote: Case 35.] Of the _Ovulidæ_, the most curious is the “Weaver’s-shuttle” (_Radius volva_), in which the shell is peculiarly beaked at both ends. It is found living on barked corals (_Gorgoniidæ_), and some of the smaller species exhibit differences of coloration, resembling the tints of the Gorgonias upon which they are found. [Sidenote: Cases 36–37.] The Naticas are mostly blind, and have a very large foot, suitable for burrowing in the sand when in quest of bivalves. They are very voracious. This is one of the groups of shells that have continued to exist from Palæozoic times. [Sidenote: Case 38.]