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 When the tide ebbs the siphons are closed and partly withdrawn."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.
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.)
Even more widely known that the clams are the oysters (Ostrea virginiana), also members of this class of molluscs. The oyster is carefully cultivated by man in many countries. It has its two shells or two shell-halves dissimilar, one valve being hollowed out to receive the body, while the other is nearly flat. The oyster is attached to the sea-bottom by the outside of the hollowed-out valve. When first hatched the young oyster swims freely by means of its cilia; after a few days it attaches itself to some solid object and grows truly oyster-like. Much care has to be taken in cultivating oysters to furnish proper conditions for growth and development. The young oysters when first attached are called "spat"; when a little older this "spat," now called "seed," may be transplanted to new beds, which are stocked in this way. In fact some beds have constantly to be thus restocked, the young oysters produced on them not finding good places to attach themselves, and so swimming away. Sometimes pieces of slate, pottery, etc., are strewed about the oyster-beds to serve as "collectors," that is, as places for the attachment of the young oysters. The extent of the acreage of the American oyster-beds is larger than that of any other country. "The Baltimore oyster-beds on the Chesapeake River and its tributaries cover 3,000 acres, and produce an annual crop of 25,000,000 bushels."
Fig. 105.—Dactylus sp., a mollusc, excavating granite. (Photograph by C. H. Snow; permission of Amer. Soc. Civil Engineers.)
