Scarcely any portion of the coast-line of the world is destitute of molluscan life, except in regions where extreme cold forbids its existence. Thus along the shores of Northern Asia there is no proper littoral fauna, the constant influence of travelling ice sweeping it all away; animal life begins at about three fathoms. But in every coast region not positively hostile to existence Mollusca make their home. Each description of habitat has its own peculiar species, which there flourish best, and exist precariously, if at all, elsewhere. Thus the sandy waste of estuaries, the loose and shingly beaches, the slimy mud-flats beset with mangroves, the low stretches of jagged rock, and even the precipitous cliffs, from whose base the sea never recedes, have all their own special inhabitants. The same is true of the deep sea, and of the ocean surface. And when we come to examine the land and fresh-water Mollusca, it is found not merely that some Mollusca are terrestrial and others fluviatile, but that certain species haunt the hills and others the valleys, some the recesses of woods and others the open meadow sides, some prefer the limestone rocks, others the sandy or clayey districts, some live only in still or gently moving waters, while others are never found except where the current is rapid and powerful. It is within the tropics that the Mollusca become most numerous, and assume their finest and quaintest forms. A tropical beach, especially where there is a good tide-fall and considerable variety of station, abounds in molluscan life to an extent which must literally be seen to be believed. The beach at Panama, to select an instance familiar to the present writer, is astonishingly rich in species, which probably amount in all to several hundreds. This is due to the immense variety of habitat. On the rocks at high-water mark, and even above them, occur _Truncatella_, _Melampus_, _Littorina_, and _Siphonaria_; where a mangrove-swamp replaces the rock, on the branches overhead are huge _Littorina_, while three species of _Cerithidea_ crawl on the mud, and _Cyrena_ and _Arca_ burrow into it. Lower down, in the rock pools at half-tide mark are _Cerithium_, _Purpura_, _Omphalius_, _Anachis_ (2 sp.), _Nassa_, and several _Crepidula_. At low-water mark of ordinary tides, under stones half buried in clean sand, are _Coecum_ and _Vitrinella_; under the blocks which rest on solid rock are _Cypraea_ (4 or 5 sp.), _Cantharus_, more _Anachis_, _Columbella_ (3 sp. including the graceful _C. harpiformis_), and _Nitidella_. Where the blocks of rock are rather muddy, _Conus_ lurks, and with it _Turritella_ and _Latirus_. Where the rocks form a flat-topped platform 2 or 3 feet high, with here and there a deep crack, huge Chitons 3 inches long conceal themselves, with two species of _Turbo_, _Purpura_, and _Clavella_. At extreme low-water mark of spring tides, _on_ the isolated rocks are _Monoceros_, _Leucozonia_, and _Vermetus_, _in_ them