Cases of this kind are admirably described by Sir Charles Lyell.[67]
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 [68] 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 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 corraline, since corallines do not live imbedded in mud.
Pages:
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113