This order has been founded by Marsh for the reception of the extraordinary Hesperornis regalis, from the Cretaceous rocks of North America. In this wonderful fossil we have a gigantic diving-bird somewhat resembling the true "Divers" or "Loons" (Colymbus), but having the jaws furnished with numerous conical recurved teeth, sunk in a deep continuous groove (fig. 352, b and d).
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 vertebrae, 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 vertebrae of the cervical and dorsal regions are of the ordinary ornithic type. 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.
Fig. 352. - Toothed Birds (Odontornithes) of the Cretaceous rocks of America. a, Left lower jaw of Ichthyomis 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.)
From the next order, the present is readily distinguished by the fact that the vertebrae resemble those of recent birds, the sternum is without a keel, the wings are rudimentary, and the teeth are implanted in a groove in the jaw and not in separate sockets.