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 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.)]

No remains of Mammals, finally, have as yet been detected in any sedimentary accumulations of Cretaceous age.

LITERATURE.

The following list comprises some of the more important works and memoirs which may be consulted with reference to the Cretaceous strata and their fossil contents:—­

 (1) ‘Memoirs of the Geological Survey of Great Britain.’
 (2) ‘Geology of England and Wales.’  Conybeare and Phillips.
 (3) ‘Geology of Yorkshire,’ vol. ii.  Phillips.
 (4) ‘Geology of Oxford and the Thames Valley.’  Phillips.
 (5) ‘Geological Excursions through the Isle of Wight.’  Mantell.
 (6) ‘Geology of Sussex.’  Mantell.
 (7) ‘Report on Londonderry,’ &c.