hich would be detached by the smallest
jar, often remain in their places. In a word, it is certain that these
animals have lived and died when the place which they now occupy was
the surface of as much of the chalk as had then been deposited; and that
each has been covered up by the layer of Globigerina mud, upon which the
creatures imbedded a little higher up have, in like manner, lived and
died. But some of these remains prove the existence of reptiles of vast
size in the chalk sea. These lived their time, and had their ancestors
and descendants, which assuredly implies time, reptiles being of slow
growth.

There is more curious evidence, again, that the process of covering up,
or, in other words, the deposit of Globigerina skeletons, did not go on
very fast. It is demonstrable that an animal of the cretaceous sea might
die, that its skeleton might lie uncovered upon the sea-bottom long
enough to lose all its outward coverings and appendages by putrefaction;
and that, after this had happened, another animal might attach itself
to the dead and naked skeleton, might grow to maturity, and might itself
die before the calcareous mud had buried the whole.

Cases of this kind are admirably described by Sir Charles Lyell.[67]
He speaks of the frequency with which geologists find in the chalk a
fossilized sea-urchin, to which is attached the lower valve of a Crania.
This is a kind of shell-fish, with a shell composed of two pieces, of
which, as in the oyster, one is fixed and the other free.

"The upper valve is almost invariably wanting, though occasionally found
in a perfect state of preservation in the white chalk at some distance.
In this case, we see clearly that the sea-urchin first lived from youth
to age, then died and lost its spines, which were carried away. Then the
young Crania adhered to the bared shell, grew and perished in its turn;
after which, the upper valve was separated from the lower, before the
Echinus [68] became enveloped in chalky mud."

A specimen in the Museum of Practical Geology, in London, still further
prolongs the period which must have elapsed between the death of the
sea-urchin, and its burial by the Globigerinae. For the outward face of
the valve of a Crania, which is attached to a sea-urchin (Micraster),
is itself overrun by an incrusting coralline, which spreads thence over
more or less of the surface of the sea-urchin. It follows that, after
the upper valve of the Crania fell off, the s