_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 that it lived principally upon fishes; but its powerful wings at the same time indicate that it was capable of prolonged flight. The tail of _Ichthyornis_ has, unfortunately, not been discovered; and it is at present impossible to say whether this resembled the tail of existing Birds, or whether it was elongated and composed of separate vertebræ, as in the Jurassic _Archoeopteryx_.