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 _Globigerinae_ 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 _Globigerinae_ 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."[32]

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
surface of the attached valve must have remained exposed long enough to
allow of the growth of the whole coralline, since corallines do