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 of the attached valve must have remained exposed long enough to allow of the growth of the whole coralline, since corallines do not live imbedded in the mud.

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 Crania 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.  Then, as the coralline could not have fixed itself if the Crania had been covered up with chalk-mud, and could not have lived had itself been so covered, it follows, that an inch of chalk mud could not have accumulated within the time between the death and decay of the soft parts of the sea-urchin and the growth of the coralline to the full size which it has attained.  If the decay of the soft parts of the sea-urchin; the attachment, growth to maturity, and decay of the Crania; and the subsequent attachment and growth of the coralline, took a year (which is a low estimate enough), the accumulation of the inch of chalk must have taken more than a year:  and the deposit of a thousand feet of chalk must, consequently, have taken more than twelve thousand years.

The foundation of all this calculation is, of course, a knowledge of the length of time the Crania and the coralline needed to attain their full size; and, on this head, precise knowledge is at present wanting.  But there are circumstances which tend to show that nothing like an inch of chalk has accumulated during the life of a Crania; and, on any probable estimate of the length of that life, the chalk period must have had a much longer duration than that thus roughly assigned to it.