26 MOLLUSCA. 
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, un- 
covered 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 per- 
mitted 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 Cardiwm have simi- 
lar habits of life. Yet Mr. Gray has enumerated 
a considerable catalogue of species, which break 
this rule, classimg 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. 
