each peculiar situation and locality has its proper kinds. The sands, the ooze and mud of harbours, the boulders and loose stones of the wave-washed beach, the sides, ledges, and pools of rocks, uncovered at the recess of every tide, are all inhabited by species peculiar to the respective locality. Some species strictly marine are able to endure protracted exposure to the air, as the Periwinkle and the Limpet, the most familiar of shell-fish, which every visitor to the sea-side habitually sees clustered on the rocks close to the limits of high-water mark.
The fresh-water Mollusca manifest a similar choice of situation, though a less latitude is permitted for its exercise. Some are peculiar to large rivers, some to estuaries, others to lakes, and yet others to small ponds and ditches.
In general the habits of one species of a genus when ascertained, are found to indicate those of all its fellows of the same genus; as for instance not only is our common Cockle (Cardium edule) a burrower in the ooze at the margin of the sea, but all other species of the genus Cardium have similar habits of life. Yet Mr. Gray has enumerated a considerable catalogue of species, which break this rule, classing them under four divisions. 1st, where species of the same genus are found in more than one kind of situation, as on land, in fresh and in salt-water; 2nd, where one or more species of a genus, most of whose species inhabit fresh-water, are found in salt or brackish water; 3rd, where, on the contrary, one or more species of a genus, whose species generally inhabit the sea, are found in fresh-water; and 4th, where the same species is found both in salt and fresh-water." 
- Philos. Trans. 1835; Part ii.