condition, that it
is impossible to say whether they differed from existing birds in any
essential character or not. In your country the development of the
cretaceous series of rocks is enormous; the conditions under which the
later cretaceous strata have been deposited are highly favourable to the
preservation of organic remains; and the researches, full of labour and
risk, which have been carried on by Professor Marsh in these cretaceous
rocks of Western America, have rewarded him with the discovery of forms
of birds of which we had hitherto no conception. By his kindness, I am
enabled to place before you a restoration of one of these extraordinary
birds, every part of which can be thoroughly justified by the more or
less complete skeletons, in a very perfect state of preservation, which
he has discovered. This _Hesperornis_ (Fig. 3), which measured between
five and six feet in length, is astonishingly like our existing 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] 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 ext