e high-water mark. I collected many of
these oysters, as also the common eatable whelks (_Buccina_), thrown up
with them, and observed that, although still living, their shells were
worn by the long attrition of sand which had passed over them as they
lay in their native bed, and which had evidently not resulted from the
mere action of the tempest by which they were cast ashore.

From these facts we learn that the union of the two parts of a bivalve
shell does not prove that it has not been transported to a distance;
and when we find shells worn, and with all their prominent parts rubbed
off, they may still have been imbedded where they grew.

_Burrowing shells._--It sometimes appears extraordinary, when we observe
the violence of the breakers on our coast, and see the strength of the
current in removing cliffs, and sweeping out new channels, that many
tender and fragile shells should inhabit the sea in the immediate
vicinity of this turmoil. But a great number of the bivalve Testacea,
and many also of the turbinated univalves, burrow in sand or mud. The
Solen and the Cardium, for example, which are usually found in shallow
water near the shore, pierce through a soft bottom without injury to
their shells; and the Pholas can drill a cavity through mud of
considerable hardness. The species of these and many other tribes can
sink, when alarmed, with considerable rapidity, often to the depth of
several feet, and can also penetrate upwards again to the surface, if a
mass of matter be heaped upon them. The hurricane, therefore, may expend
its fury in vain, and may sweep away even the upper part of banks of
sand or mud, or may roll pebbles over them, and yet these Testacea may
remain below secure and uninjured.

_Shells become fossil at considerable depths._--I have already stated
that, at the depth of 950 fathoms, between Gibraltar and Ceuta, Captain
Smith found a gravelly bottom, with fragments of broken shells, carried
thither probably from the comparatively shallow parts of the neighboring
straits, through which a powerful current flows. Beds of shelly sand
might here, in the course of ages, be accumulated several thousand feet
thick. But, without the aid of the drifting power of a current, shells
may accumulate in the spot where they live and die, at great depths from
the surface, if sediment be thrown down upon them; for even in our own
colder latitudes, the depths at which living marine animals abound is
very consider