This order has been founded by Marsh for the reception of two remarkable birds, which he has named Ichthyornis dispar and Apatornis celer, both from the Cretaceous rocks of North America.

In Ichthyornis dispar, which may be taken as the type of the order, the teeth (fig. 352, a) were sunk in distinct sockets, and were "small, compressed, and pointed, and all of those preserved are similar. Those in the lower jaw number about twenty in each ramus, and are all more or less inclined backwards. . . . The maxillary teeth appear to have been equally numerous, and essentially the same as those in the mandible. The skull was of moderate size, and the eyes placed well forward. The lower jaws are long and slender, and the rami were not closely united at the symphysis. . . . The jaws were apparently not encased in a horny sheath.

" The scapular arch, and the bones of the wings and legs, all conform closely to the true ornithic type. The wings were large in proportion to the legs, and the humerus had an extended radial crest. The metacarpals were united, as in ordinary birds. The bones of the posterior extremities resemble those of swimming birds. The vertebrae (see fig. 352, c and c') were all biconcave, the concavities at each end of the centra being distinct and nearly alike. Whether the tail was elongated cannot at present be determined; but the last vertebra of the sacrum was unusually large.

" The bird was fully adult, and about as large as a pigeon. With the exception of the skull, the bones do not appear to have been pneumatic, though most of them are hollow. The species was carnivorous, and probably aquatic." (Marsh.)

Apatornis agrees with Ichthyornis in most of the above characters, but the structure of its jaws is not fully known. It follows from the above that the order Odontotormae is characterised by the possession of distinct teeth sunk in separate sockets in the jaw and not in a continuous groove, by the fact that the vertebrae are biconcave, and by the possession of a carinate sternum and well-developed wings.