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Other fossil forms like Hesperornis and Ichthyornis, whose remains occur in the strata of a later date, fill in the gap between Archæopteryx and the birds at the present time, for among other things they possess teeth which indicate their origin from forms like Archæopteryx, while in other respects they are far nearer the birds of later epochs.
Ichthyornis also differed in the fact that its vertebrae have not the peculiar characters of the vertebrae of existing and of all known tertiary birds, but were concave at each end. This discovery leads us to make a further modification in the definition of the group of birds, and to part with another of the characters by which almost all existing birds are distinguished from reptiles.
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.
Ichthyornis also differed in the fact that its vertebræ have not the peculiar characters of the vertebræ of existing and of all known tertiary birds, but were concave at each end. This discovery leads us to make a further modification in the definition of the group of birds, and to part with another of the characters by which almost all existing birds are distinguished from reptiles.
The latter also has such very small, almost rudimentary wings, that it must have been chiefly a swimmer and a diver like a Penguin; while Ichthyornis has strong wings and no doubt possessed corresponding powers of flight.
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.
A more reptilian character in the Ichthyornis group is the fact that, unlike any modern bird, but like their reptile ancestors, they had biconcave vertebrae. The brain was relatively poor. We are still dealing with a type intermediate in some respects between the reptile and the modern bird. The gannets, cormorants, and pelicans are believed to descend from some branch of this group.
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.
The latter also has such very small, almost rudimentary, wings, that it must have been chiefly a swimmer and a diver, like a Penguin; while Ichthyornis has strong wings and no doubt possessed corresponding powers of flight.
One of these lines the pelicans, cormorants, etc. seems to be a continuation of the Ichthyornis type of the Cretaceous, with the Odontopteryx as an Eocene offshoot; the divers, penguins, grebes, and petrels represent another ancient stock, which may be related to the Hesperornis group of the Cretaceous. Dr.
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