_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 maximum rate at which the chalk can have accumulated,
and thus to arrive at the minimum duration of the chalk period. Suppose
that the valve of the _Cronia_ upon which a coralline has fixed itself in
the way just described, is so attached to the sea-urchin that no part of
it is more than an inch above the face upon which the sea-urchin rests.