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. [Illustration: FIG. 3--HESPERORNIS REGALIS (Marsh).] The same formation has yielded another bird _Ichthyornis_ (Fig. 5), which also possesses teeth; but the teeth are situated in distinct sockets, while those of _Hesperornis_ are not so lodged. 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. _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. [Illustration: FIG. 4.--HESPERORNIS REGALIS (Marsh). Side and upper views of half the lower jaw; side and end views of a vertebra and a separate tooth.] 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. These so-called slates are composed of a fine grained calcareous mud which has hardened into lithographic stone, and in which organic remains are almost as well preserved as they would be if they had been imbedded in so much plaster of Paris. They have yielded the _Archaeopteryx_, the existence of which was first made known by the finding of a fossil feather, or rather of the impression of one. It is wonderful enough that such a perishable thing as a feather, and nothing more, should be discovered; yet for a long time, nothing was known of this bird except its feather. But by and by a solitary skeleton was discovered which is now in the British Museum. The skull of this solitary specimen is unfortunately wanting, and it is therefore uncertain whether the _Archaeopteryx_ possessed teeth or not.[2] But the remainder of the skeleton is so well preserved as to leave no doubt respecting the main features of the animal, which are very singular. The feet are not only altogether bird-like, but have the special characters of the feet of perching birds, while the body had a clothing of true feathers. Nevertheless, in some other respects, _Archaeopteryx_ is unlike a bird and like a reptile. There is a long tail composed of many vertebrae.