[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 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 vertebrae, as in the Jurassic Archoeopteryx.

Still more wonderful than Ichthyornis is the marvellous bird described by Marsh under the name of Hesperornis regalis.  This presents us with a gigantic diving bird, somewhat resembling the existing “Loons” (Colymbus), but agreeing with Ichthyornis in having the jaws furnished with conical, recurved, pointed teeth (fig. 212, b).  Hence these forms are grouped together in a new sub-class, under the name of Odontornithes or “Toothed Birds.”  The teeth of Hesperornis (fig. 212, d) resemble those of Ichthyornis in their general form; but instead of being sunk in distinct sockets, they are simply implanted in a deep continuous groove in the bony substance of the jaw.  The front of the upper jaw does not carry teeth, and was probably encased