rae 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. 4.--HESPERORNIS REGALIS (Marsh).

Side and upper views of half the lower jaw; side and end views of a
vertebra and a separate tooth.]

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. But by and by a solitary skeleton was
discovered which is now in the British Museum. The skull of this
solitary specimen is unfortunately wanting, and it is therefore
uncertain whether the _Archaeopteryx_ possessed teeth or not.[2] But the
remainder of the skeleton is so well preserved as to leave no doubt
respecting the main features of the animal, which are very singular. The
feet are not only altogether bird-like, but have the special characters
of the feet of perching birds, while the body had a clothing of true
feathers. Nevertheless, in some other respects, _Archaeopteryx_ is unlike
a bird and like a reptile. There is a long tail composed of many
vertebrae. The structure of the wing differs in some very remarkable
respects from that which it presents in a true bird. In the latter, the
end of the wing answers to the thumb and two fingers of my hand; but the
metacarpal bones, or those which answer to the bones of the fingers
which lie in the palm of the hand, are fused together into one mass; and
the whole apparatus, except the last joints of the thumb, is bound up in
a sheath of