The fresh-water mussel which we have studied is an example of the bivalve molluscs. The members of this class show a range in size from the little fresh-water _Cyclas_ about 1 cm. long to the giant clam of the Indian and Pacific islands "which is sometimes 60 cm. (2 feet) in length and 500 pounds in weight." They show also some variety in the form and appearance of the shell, but not anything like the degree of variety shown by the shells of the Gastropods. The edible clams are of several different species. The hard-shell clam (_Venus mercenaria_), or "quohog" as it is often called, is found along the Atlantic coast from Texas to Cape Cod. It is "common on sandy shores, living chiefly on the sandy and muddy plots, just beyond low-water mark.... It also inhabits estuaries, where it most abounds. It burrows a short distance below the surface, but is frequently found crawling at the surface with the shell partly exposed." The shells of this edible clam are white. The soft-shell clam (_Mya arenaria_), "the clam _par excellence_, which figures so largely in the celebrated New England clam-bake, is found in all the northern seas of the world.... All along the coasts of the eastern States, every sandy shore, every mud flat, is full of them, and from every village and hamlet the clam-digger goes forth at low tide to dig these esculent bivalves. The clams live in deep burrows in the firm mud or sand, the shells sometimes being a foot or fifteen inches beneath the surface. When the flats are covered with water his clamship extends his long siphons up through the burrow to the surface of the sand, and through one of these tubes the water and its myriads of animalcules is drawn down into the shell, furnishing the gills with oxygen and the mouth with food, and then the water charged with carbonic acid and fæcal refuse is forced out of the other siphon. When the tide ebbs the siphons are closed and partly withdrawn." Ocean clams and mussels have furnished food for man for ages, and along coasts are found here and there great mounds made of heaps of clam-shells which have become covered over with soil and vegetation. Such mounds are the old feasting-places of the early coast inhabitants, and the archæologist often finds in these "kitchen-middens," as they are called, various relics of the early natives of the continent. [Illustration: FIG. 104.--A group of marine Pacific Coast molluscs; in upper left-hand corner, _Purpura saxicola_; next to the right, _Littorina scutulata_; farthest to right, limpets, _Acmara spectrum_; left-hand lower corner, _Mytilus californianus_; in right-hand lower corner the black shells just above the large clam-shell, _Chlorostomum funebrale_. (From living specimens in a tide pool in the Bay of Monterey, California.)]