as this, it might have been said that birds were characterized by the absence of teeth; but the discovery of a bird that had teeth shows at once that there were ancient birds that, in that particular respect, approached reptiles more nearly than any existing bird does.
The same rocks have yielded another bird (Ichthiyornis), which also has teeth in its jaws, the teeth in this case being situated in distinct sockets, while those of Hesperornis were not so lodged. The latter also had very small wings, while Ichthyornis has strong wings. Ichthyornis also differed in the fact that the joints of its backbone—its vertebræ had not the peculiar character that the vertebræ of existing birds have, but were concave at each end. This discovery leads us to make another modification in the definition of the group of birds, and to part with another of the characters by which they are distinguished from reptiles. We know nothing whatever of birds older than these until we come down to the Jurassic period, and from rocks of that age we have a single bird which was first made known by the finding of a fossil feather. It was thought wonderful that such a perishable thing as a feather should be discovered and nothing more, and so it was; and for a long time nothing was known of this bird except its feather. But, by-and-by one solitary specimen was discovered, which is now in the British Museum. That solitary specimen is unfortunately devoid of its head; but there is this wonderful peculiarity about the creature that, so far as its feet are known, it has all the characters of a bird, all those peculiarities by which a bird is distinguished from a reptile. Nevertheless, in other respects, it is unlike a bird and like a reptile. There is a long series of caudal vertebræ. The wing differs in some very remarkable respects from the structure it presents in a true bird. In a true bird the wing answers to these three fingers—the thumb and two fingers of my hand—the metacarpal bones are fused together into one mass—and the whole apparatus except the thumb is bound up in a sheath of integument, and the edge of the hand carries the principal quill-feathers. It is in that way that the bird's wing becomes an instrument of flight. In the Archæopteryx, the upper-arm bone is like that of a bird; these two forearm bones are more or less like those of a bird, but the fingers are not bound together—they are free, and they are all terminated by strong claws, not like such as are sometimes found in birds, but by such as reptiles possess, so that in the Archæopteryx you have an animal which, to a certain extent, occupies a midway place between a bird and a reptile. It is a bird so far as its foot and sundry other parts of its skeleton are concerned; it is essentially and thoroughly a bird in the fact that it possesses feathers, but it is much more properly a reptile in the fact that what represents the hand has separate bones resembling those which terminate the fore-limb of a reptile. Moreover, it had a long tail with a fringe of feathers on each side. All these cases, so far as