le 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."[4]

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 _Globigerinoe_. 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 not live embedded in mud.[4]

[Footnote 4: _Elements of Geology_, by Sir Charles Lyell, Bart. F.B.S.,
p. 23.]

The progress of knowledge may, one day, enable us to deduce from such
facts as these the ma