[Illustration: FIG. 19.--Some British Marine Bivalve Molluscs.
a, The smaller Piddock, Pholas parva, which bores into chalk, clay, and hard rock. Natural size.
b, The Razor-shell, Solen siliqua. The muscular foot is seen protruding from the shell. One-third the natural size, linear.
d, Cardium echinatum. Two-thirds the natural size, linear.
e, Pinna pectinata, the "cappy longy." One-fifth of the natural size, linear.]
Though "shells" are often numerous on parts of the seashore, some beaches (as, for instance, at Falmouth, at the mouth of the Eden of St. Andrews, and at Herm in the Channel Islands) being so placed in regard to the currents and waves of the sea that great quantities of shells of dozens of species are thrown up, and even "make up" the beach, yet there are not so very many Molluscs which live commonly on the shore between tide-marks. The shells which are accumulated as shell-beaches have come from animals which lived in quantity at depths of ten or twenty fathoms, whence they can be brought up alive by the dredge. There are, however, certain bivalves and certain univalves which are commonly to be found in the living state between tide-marks. You will not find the oyster there on our own coast, but in Australia they have picnic parties where every guest provides himself with a hammer and a bottle of vinegar and a pepper-pot, and at low tide proceeds to chip the oysters off the rocks on which they grow tightly fixed, and to eat them "right away" before they have time to lose their good temper and sweetness! In Jamaica they show you oysters apparently growing on trees high up in the air, but they are dead, having attached themselves to the branches of a young tree which dipped into the water. Once fixed there, they were unable to move as the tree grew and carried them up with its branches above the sea-level.
The only bivalve at all common and visible to the eye between tide-marks is the common or edible sea-mussel, which is attached in purple clusters to the rocks (as in North Cornwall), or forms a wide-spreading pavement, called a "scalp," of as much as an acre in extent, on which thousands of mussels lie side by side. But by digging in the sand and mud between tides there are other living bivalves to be found, which burrow more or less deeply. The razor-shell (Fig. 19, b) is one of these (see p. 80). Often (as at Teignmouth and Barmouth) we find "cockles" buried in the sand, and those delicate, smooth bivalves not an inch long, white outside and purple within, which are made into soup at Naples and are called "vongoli," but have no English name. Other "clams" (Tapes, which is eaten in France, even in Paris, and Mya, and Scrobicularia which lives in black mud) may be dug up, but they are devoid of English names because we do not eat them; hence I have to speak of them by their Latin scientific names. As to univalves, there are three which are found almost everywhere on our coasts where there are rocks, namely, the periwinkles (one species of which actually lives above high tide-mark), the limpet, and the dog-whelk. A small species of top-shell or trochus is also very common, and so is the chiton, or armadillo-shell, which, though really the most primitive and nearest representative of the ancestors of all univalve molluscs, yet has its own shell of a very peculiar character (sometimes with very minute eyes--true eyes--dotted about on it), and always divided transversely to its length (not right and left) into eight separate pieces, which, indeed, seem to be really separate, independent little shells, corresponding to eight segments like the segments of a shrimp or an earth-worm.