hidia_) have
as yet been detected in the Cretaceous rocks; and this order
does not appear to have come into existence till the Tertiary
period. Lastly, true Crocodiles are known to have existed in
considerable numbers in the Cretaceous period. The oldest of
these occur in the fresh-water deposit of the Wealden; and they
differ from the existing forms of the group in the fact that the
bodies of the vertebrae, like those of the Jurassic Crocodiles,
are bi-concave, or hollowed out at both ends. In the Greensand
of North America, however, occur the remains of Crocodiles which
agree with all the living species in having the bodies of the
vertebrae in the region of the back hollowed out in front and
convex behind.

[Illustration: Fig. 211.--Carapace of _Chelone Benstedi_. Lower
Chalk. (After Owen.)]

_Birds_ have not hitherto been shown, with certainty, to have
existed in Europe during the Cretaceous period, except in a few
instances in which fragmentary remains belonging to this class
have been discovered. The Cretaceous deposits of North America
have, however, been shown by Professor Marsh to contain a
considerable number of the remains of Birds, often in a state
of excellent preservation. Some of these belong to Swimming or
Wading Birds, differing in no point of special interest from
modern birds of similar habits. Others, however, exhibit such
extraordinary peculiarities that they merit more than a passing
notice. One of the forms in question constitutes the genus
_Ichthyornis_ of Marsh, the type-species of which (_I. Dispar_)
was about as large as a Pigeon. In two remarkable respects, this
singular Bird differs from all known living members of the class.
One of these respects concerns the jaws, both of which exhibit the
Reptilian character of being armed with numerous small pointed
_teeth_ (fig. 212, a), sunk in distinct sockets. No existing
bird possesses teeth; and this character forcibly recalls the
Bird-like Pterosaurs, with their toothed jaws. _Ichthyornis_,
however, possessed fore-limbs constructed strictly on the type
of the "wing" of the living Birds; and it cannot, therefore, be
separated from this class. Another extraordinary peculiarity
of _Ichthyornis_ is, that the bodies of the _vertebrie_ (fig.
212, c) were _bi-concave_, as is the case with many extinct
Reptiles and almost all Fishes, but as does not occur in any
living Bird. There can be little doubt that _Ichthyornis_ was
aquatic in its habits, and