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.
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