and still have teeth in the jaws. They are of two leading
types, of which the Ichthyornis and Hesperornis are the standard
specimens. The Ichthyornis was a small, tern-like bird with the power of
flight strongly developed, as we may gather from the frame of its wings
and the keel-shaped structure of its breast-bone. Its legs and feet were
small and slender, and its long, slender jaws had about twenty teeth on
each side at the bottom. No modern bird has teeth; though the fact that
in some modern species we find the teeth appearing in a rudimentary
form is another illustration of the law that animals tend to reproduce
ancestral features in their development. A more reptilian character in
the Ichthyornis group is the fact that, unlike any modern bird, but like
their reptile ancestors, they had biconcave vertebrae. The brain was
relatively poor. We are still dealing with a type intermediate in
some respects between the reptile and the modern bird. The gannets,
cormorants, and pelicans are believed to descend from some branch of
this group.

The other group of Cretaceous birds, of the Hesperornis type, show an
actual degeneration of the power of flight through adaptation to an
environment in which it was not needed, as happened, later, in the kiwi
of New Zealand, and is happening in the case of the barn-yard fowl.
These birds had become divers. Their wings had shrunk into an abortive
bone, while their powerful legs had been peculiarly fitted for diving.
They stood out at right angles to the body, and seem to have developed
paddles. The whole frame suggests that the bird could neither walk nor
fly, but was an excellent diver and swimmer. Not infrequently as large
as an ostrich (five to six feet high), with teeth set in grooves in
its jaws, and the jaws themselves joined as in the snake, with a great
capacity of bolting its prey, the Hesperornis would become an important
element in the life of the fishes. The wing-fingers have gone, and the
tail is much shortened, but the grooved teeth and loosely jointed jaws
still point back to a reptilian ancestry.

These are the only remains of bird-life that we find in the Mesozoic
rocks. Admirably as they illustrate the evolution of the bird from the
reptile, they seem to represent a relatively poor development and spread
of one of the most advanced organisms of the time. It must be understood
that, as we shall see, the latter part of the Chalk period does not
belong to the depression,