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. Fig. 3--Hesperornis regalis (Marsh) 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.) 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. Figure. 5--Ichthyornis Dispar (Marsh). Side and upper views of half the lower jaw; and side and end views of a vertebra.