Thus it is that corals, crustacea, insects, mollusks, and a few other
kinds of lower forms constitute the greater mass of invertebrate
palæontological materials because of their supporting structures of one
kind or another. Perhaps the skeletal remains of the vertebrates of the
past provide the student of fossils with his best facts, on account of the
resistant nature of the bones themselves, and because the backboned
animals are relatively modern; then, too, the rocks in which their remains
occur have not been so much altered by geological agencies, or buried so
deeply under the strata formed later. Of course only the hardest kinds of
shells would remain as such after their burial in materials destined to
turn into rock; in the majority of cases, an entombed bone is infiltrated
or replaced by various mineral substances so that in time little or
nothing of the original thing would remain, though a mold or a cast would
persist.

But even if an animal of the past possessed hard structures, it must have
satisfied certain limited conditions to have its remains prove serviceable
to students of to-day. A dead mammal must fall upon ground that has just
the right consistency to receive it; if the soil is too soft, its several
parts will be separated and scattered as readily as though it had fallen
upon hard ground where it would be torn to pieces by carnivorous animals.
The dead body must then be covered up by a blanket of silt or sand like
that which would be deposited as the result of a freshet. If a skeleton is
too greatly broken up or scattered, it may be difficult or even impossible
for its discoverer to piece together the various fragments and assemble
them in their original relations. Very few individuals have been so buried
and preserved as to meet the conditions for the formation of an ideal
fossil. To realize how little may be left of even the most abundant of
higher organisms, we have only to recall that less than a century ago
immense herds of bison and wild horses roamed the Western plains, but very