definitions of the classes of birds and reptiles. Before the discovery
of _Hesperornis_, the definition of the class Aves based upon our
knowledge of existing birds, might have been extended to all birds; it
might have been said that the absence of teeth was characteristic of the
class of birds; but the discovery of an animal which, in every part of
its skeleton, closely agrees with existing birds, and yet possesses
teeth, shows that there were ancient birds which, in respect of
possessing teeth, approached reptiles more nearly than any existing bird
does, and, to that extent, diminishes the _hiatus_ between the two
classes.

[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.)]

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 vertebræ have not the
peculiar characters of the vertebræ 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).