_a_, siphon; _b_, foot; _c_, tentacles; _d_, eyes; _e_, operculum. ] [Sidenote: Cases 70–76.] The family of _Muricidæ_, or “Rock-shells,” is another extensive group, containing many very handsome and peculiar forms. The animals of this family have a long proboscis, at the end of which is the spiny tongue (radula), and which is retractile within the body. The true Murices produce at intervals ribs or varices, which in some species are ornamented with long spines or foliations, and which indicate periods of growth, but of what duration we do not know. They are all carnivorous, feeding chiefly on other Mollusca, boring through the shells of bivalves with their spiny tongue, and slowly devouring the unfortunate inhabitant piecemeal. From certain species of _Murex_ (_M. brandaris_, &c.) found in the Mediterranean, the ancients manufactured the celebrated Tyrian purple dye. [Sidenote: Cases 74–75.] The “Purples” (_Purpura_) are found between tide-marks all over the world. _Magilus_, belonging to the family _Coralliophilidæ_ (Case 77), is found among coral-reefs in tropical seas, and has the remarkable habit of lengthening the aperture of its shell into an elongate tube, in order to keep pace with the growth of the coral, and to prevent its being overgrown and killed. [Sidenote: Cases 78–81.] The “Volutes” (_Volutidæ_) are a group of shells also much sought after by shell-collectors. Some of these attain to a very large size, the animals inhabiting them being enormous. The Boat-shells (_Cymba_) and Melons (_Cymbium_) are ovo-viviparous, the young being carried about by the parent until they are an inch in length. Volutes are found chiefly in the warmer parts of the Atlantic and Indo-Pacific Oceans, and occur in the greatest variety on the coasts of Australia. [Sidenote: Cases 81–83.] The Olives (_Olividæ_) are common in most tropical seas, and are remarkable for their beautiful polish and various patterns of colouring. In structure and form they are very similar to each other. They burrow in sand in quest of bivalves for food, and some species are said to have the power of swimming by expanding the lobes of the foot. [Sidenote: Case 84.] The Harps (_Harpidæ_) form a small well-marked group, of which probably nearly all the existing species have been discovered. The animals inhabiting these beautiful shells are also brightly coloured. They have the remarkable power of casting off a portion of the foot when disturbed. The species are known from the Indo-Pacific Ocean, the west coast of Central America, and West Africa. [Sidenote: Cases 85–87.]