ent, diminishes the
_hiatus_ between the two classes.

Fig. 3--Hesperornis regalis (Marsh)

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.)

The same formation has yielded another bird, _Ichthyornis_ (Fig. 5),
which also possesses teeth; but the teeth are situated in distinct
sockets, while those of _Hesperornis_ are not so lodged. The latter also
has such very small, almost rudimentary wings, that it must have been
chiefly a swimmer and a diver like a Penguin; while _Ichthyornis_ has
strong wings and no doubt possessed corresponding powers of flight.
_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.

Figure. 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. 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