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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.
In possessing true teeth, the Hesperornis differs from every existing bird, and from every bird yet discovered in the tertiary formations, the tooth-like serrations of the jaws in the Odontopteryx of the London clay being mere processes of the bony substance of the jaws, and not teeth in the proper sense of the word.
They are widely removed from our modern types of birds, 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.
By his kindness, I am enabled to place before you a restoration of one of these extraordinary birds, every part of which can be thoroughly justified by the more or less complete skeletons, in a very perfect state of preservation, which he has discovered. But Hesperornis differs from all existing birds, and so far resembles reptiles, in one important particular it is provided with teeth.
The Ichthyornis type was a tern-like flying bird, with socketed teeth and biconcave vertebrae like the reptile, but otherwise fully evolved into a bird. Its line is believed to survive in the gannets, cormorants, pelicans, and frigate-birds of to-day. The less numerous Hesperornis group were large and powerful divers.
Before the discovery of Hesperornis, the definition of the class Aves based upon our knowledge of existing birds might have been extended to all birds; it might have been said that the absence of teeth was characteristic of the class of birds; but the discovery of an animal which, in every part of its skeleton, closely agrees with existing birds, and yet possesses teeth, shows that there were ancient birds which, in respect of possessing teeth, approached reptiles more nearly than any existing bird does, and, to that extent, diminishes the hiatus between the two classes.
Apart from the few fragmentary remains from the English greensand, to which I have referred, the mesozoic rocks, older than those in which Hesperornis and Ichthyornis have been discovered have afforded no certain evidence of birds, with the remarkable exception of the Solenhofen slates.
In possessing true teeth, the Hesperornis differs from every existing bird, and from every bird yet discovered in the tertiary formations, the tooth-like serrations of the jaws in the Odontopteryx of the London clay being mere processes of the bony substance of the jaws, and not teeth in the proper sense of the word.
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