particular--it is provided with teeth. The long jaws are armed with
teeth which have curved crowns and thick roots (Fig. 4), and are not set
in distinct sockets, but are lodged in a groove. In possessing true
teeth, the _Hesperornis_ differs from every existing bird, and from
every bird yet discovered in the tertiary formations, the tooth-like
serrations of the jaws in the _Odontopteryx_ of the London clay being
mere processes of the bony substance of the jaws, and not teeth in the
proper sense of the word. In view of the characteristics of this bird we
are therefore obliged to modify the 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. 3--HESPERORNIS REGALIS (Marsh).]

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