Mollusks are found in every part of the world. The arctic seas possess their own characteristic faunæ, the more temperate waters of Europe and of America contain their own peculiar genera and species, and the warm waters of the tropic seas furnish the conditions favorable to the life of an immense number of characteristic forms. Again, there are the littoral species, that live only between tide-marks and are therefore exposed to the air for a number of hours each day; some of these live just about high-tide mark and have become almost terrestrial in their habits, while others must be sought at the point of lowest tide, where for only a short time each day they are deprived of their natural element. Then there are the shallow-water forms, which never appear above low-tide mark save when a neap tide surprises them; their range in depth extends to about the hundred-fathom line, more or less. Beyond this depth to several hundred fathoms other characteristic forms appear, and from the more profound depths of mid-ocean the dredge has brought to light a host of curious and interesting species.
It is the temperature of the water rather than the depth that appears to influence the distribution of marine mollusks. Thus, certain species whose natural home is in the shallow waters of the Arctic Ocean have been taken in very deep waters off the southern coast of the United States, the temperature conditions in both stations being substantially the same. This fact, however, must not be too freely accepted as establishing a principle. Some [pg309] shallow-water forms have been taken at great depths, but in general the abyssal fauna is a peculiar one that cannot well be marked off into geographical provinces.
The east coast of America is divided into several molluscan faunal regions. A series of very-cold-water forms belonging to a circumpolar region, called the "arctic province," are found as far south as Newfoundland. On the New England coast a number of these arctic species are also found, urged south by the influence of the cold Labrador current. The most characteristic genera belonging to this arctic fauna which are found upon the Maine and Massachusetts coasts are /Buccinum/, /Chrysodomus/, /Sipho/, /Trophon/, /Bela/, /Velutina/, /Trichotropis/, /Lacuna/, /Margarita/, /Pecten/, /Leda/, /Yoldia/, /Astarte/, and /Mya/. Examples of all these genera are encountered as far south as Cape Cod.