that time, the precise shape of the bones was retained.
If that sandstone had remained soft a little longer, we should have
known nothing whatsoever of the existence of the reptile whose bones it
had encased.

How certain it is that a vast number of animals which have existed
at one period on this earth have entirely perished, and left no trace
whatever of their forms, may be proved to you by other considerations.
There are large tracts of sandstone in various parts of the world, in
which nobody has yet found anything but footsteps. Not a bone of any
description, but an enormous number of traces of footsteps. There is no
question about them. There is a whole valley in Connecticut covered with
these footsteps, and not a single fragment of the animals which made
them has yet been found. Let me mention another case while upon that
matter, which is even more surprising than those to which I have yet
referred. There is a limestone formation near Oxford, at a place called
Stonesfield, which has yielded the remains of certain very interesting
mammalian animals, and up to this time, if I recollect rightly, there
have been found seven specimens of its lower jaws, and not a bit of
anything else, neither limb-bones nor skull, or any part whatever; not
a fragment of the whole system! Of course, it would be preposterous
to imagine that the beasts had nothing else but a lower jaw!
The probability is, as Dr. Buckland showed, as the result of his
observations on dead dogs in the river Thames, that the lower jaw, not
being secured by very firm ligaments to the bones of the head, and being
a weighty affair, would easily be knocked off, or might drop away from
the body as it floated in water in a state of decomposition. The jaw
would thus be deposited immediately, while the rest of the body would
float and drift away altogether, ultimately reaching the sea, and
perhaps becoming destroyed. The jaw becomes covered up and preserved
in the river silt, and thus it comes that we have such a curious
circumstance as that of the lower jaws in the Stonesfield slates. So
that, you see, faulty as these layers of stone in the earth's crust
are, defective as they necessarily are as a record, the account of
contemporaneous vital phenomena presented by them is, by the necessity
of the case, infinitely more defective and fragmentary.

It was necessary that I should put all this very strongly before you,
because, otherwise, you might have been led to thin