which 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. 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 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 Globigeringae. For the outward
face of the valve of a Crania, which is attached to a sea-urchin
(Micrastor), 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
surface