_Ichthyornis_ also differed in the fact that its vertebrae have not the
peculiar characters of the vertebrae of existing and of all known
tertiary birds, but were concave at each end. This discovery leads us to
make a further modification in the definition of the group of birds, and
to part with another of the characters by which almost all existing
birds are distinguished from reptiles.
[Illustration: FIG. 5.--ICHTHYORNIS DISPAR (Marsh).
(Side and upper views of half the lower jaw; and side and end views of a
vertebra.)]
Apart from the few fragmentary remains from the English greensand, to
which I have referred, the mesozoic rocks, older than those in which
_Hesperornis_ and _Ichthyornis_ have been discovered have afforded no
certain evidence of birds, with the remarkable exception of the
Solenhofen slates. These so-called slates are composed of a fine grained
calcareous mud which has hardened into lithographic stone, and in which
organic remains are almost as well preserved as they would be if they
had been imbedded in so much plaster of Paris. They have yielded the
_Archaeopteryx_, the existence of which was first made known by the
finding of a fossil feather, or rather of the impression of one. It is
wonderful enough that such a perishable thing as a feather, and nothing
more, should be discovered; yet, for a long time, nothing was known of
this bird except its feather.
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