xisting divers
or grebes in a great many respects; so like them indeed that, had the
skeleton of _Hesperornis_ been found in a museum without its skull, it
probably would have been placed in the same group of birds as the divers
and grebes of the present day.[1]

[Illustration: FIG. 3.--HESPERORNIS REGALIS (Marsh).]

But _Hesperornis_ differs from all existing birds, and so far resembles
reptiles, in one important 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. 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 modifica