Still more wonderful than _Ichthyornis_ is the marvellous bird described by Marsh under the name of _Hesperornis regalis_. This presents us with a gigantic diving bird, somewhat resembling the existing "Loons" (_Colymbus_), but agreeing with _Ichthyornis_ in having the jaws furnished with conical, recurved, pointed teeth (fig. 212, b). Hence these forms are grouped together in a new sub-class, under the name of _Odontornithes_ or "Toothed Birds." The teeth of _Hesperornis_ (fig. 212, d) resemble those of _Ichthyornis_ in their general form; but instead of being sunk in distinct sockets, they are simply implanted in a deep continuous groove in the bony substance of the jaw. The front of the upper jaw does not carry teeth, and was probably encased in a horny beak. The breast-bone is entirely destitute of a central ridge or keel, and the wings are minute and quite rudimentary; so that _Hesperornis_, unlike _Ichthyornis_, must have been wholly deprived of the power of flight, in this respect approaching the existing Penguins. The tail consists of about twelve vertebræ, of which the last three or four are amalgamated to form a flat terminal mass, there being at the same time clear indications that the tail was capable of up and down movement in a vertical plane, this probably fitting it to serve as a swimming-paddle or rudder. The legs were powerfully constructed, and the feet were adapted to assist the bird in rapid motion through the water. The known remains of _Hesperornis regalis_ prove it to have been a swimming and diving bird, of larger dimensions than any of the aquatic members of the class of Birds with which we are acquainted at the present day. It appears to have stood between five and six feet high, and its inability to fly is fully compensated for by the numerous adaptations of its structure to a watery life. Its teeth prove it to have been carnivorous in its habits, and it probably lived upon fishes. It is a curious fact that two Birds agreeing with one another in the wholly abnormal character of possessing teeth, and in other respects so entirely different, should, like _Ichthyornis_ and _Hesperornis_, have lived not only in the same geological period, but also in the same geographical area; and it is equally curious that the area inhabited by these toothed Birds should at the same time have been tenanted by winged and bird-like Reptiles belonging to the toothed genus _Pterodactylus_ and the toothless genus _Pteranodon_. [Illustration: Fig. 212.--Toothed Birds (_Odontornithes_) of the Cretaceous Rocks of America. a. Left lower jaw of _Ichthyornis dispar_, slightly enlarged; b, Left lower jaw of _Hesperornis regalis_, reduced to nearly one-fourth of the natural size; c. Cervical vertebra of _Ichthyornis dispar_, front view, twice the natural size; c', Side view of the same; d, Tooth of _Hesperornis regalis_, enlarged to twice the natural size. (After Marsh.)]