282 HISTORICAL PALZONTOLOGY. 
recurved, pointed teeth (fig. 212, 6). Hence these forms are 
grouped together in a new sub-class, under the name of Odon- 
tornithes or “'Toothed Birds.” The teeth of Hesperornis (fig. 
212, @) resemble those of /chthyornis in their general form ; 
Fig, 212.—Toothed Birds (Odontornithes) of the Cretaceous Rocks of America. a, 
Left lower jaw of /chthyornis dispar, slightly enlarged ; 4, Left lower jaw of Hesferornis 
vegalis, reduced to nearly one-fourth of the natural size; c, Cervical vertebra of /chthyoriis 
dispar, front view, twice the natural size; c’, Side view of the same ; d, Tooth of Hesfer- 
ornis regalis, enlarged to twice the natural size. (After Marsh.) 
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 Aesferornis, 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 vertebrz, 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 proba- 
bly 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 Hesferornis regalis prove it to have been a swim- 
ming and diving bird, of larger dimensions than any of the 
