the existing forms of the group in the fact that the bodies of the vertebræ, like those of the Jurassic Crocodiles, are bi-concave, or hollowed out at both ends. In the Greensand of North America, however, occur the remains of Crocodiles which agree with all the living species in having the bodies of the vertebræ in the region of the back hollowed out in front and convex behind.

Birds have not hitherto been shown, with certainty, to have existed in Europe during the Cretaceous period, except in a few instances in which fragmentary remains belonging to this class have been discovered. The Cretaceous deposits of North America have, however, been shown by Professor Marsh to contain a considerable number of the remains of Birds, often in a state of excellent preservation. Some of these belong to Swimming or Wading Birds, differing in no point of special interest from modern birds of similar habits. Others, however, exhibit such extraordinary peculiarities that they merit more than a passing notice. One of the forms in question constitutes the genus Ichthyornis of Marsh, the typespecies of which (I. dispar) was about as large as a Pigeon. In two remarkable respects, this singular Bird differs from all known living members of the class. One of these respects concerns the jaws, both of which exhibit the Reptilian character of being armed with numerous small pointed teeth (fig. 212, a), sunk in distinct sockets. No existing bird possesses teeth; and this character forcibly recalls the Bird-like Pterosaurs, with their toothed jaws. Ichthyornis, however, possessed fore-limbs constructed strictly on the type of the "wing" of the living Birds; and it cannot, therefore, be separated from this class. Another extraordinary peculiarity of Ichthyornis is, that the bodies of the vertebræ (fig. 212, c) were bi-concave, as is the case with many extinct Reptiles and almost all Fishes, but as does not occur in any living Bird. There can be little doubt that Ichthyornis was aquatic in its habits, and that it lived principally upon fishes; but its powerful wings at the same time indicate that it was capable of prolonged flight. The tail of Ichthyornis has, unfortunately, not been discovered; and it is at present impossible to say whether this resembled the tail of existing Birds, or whether it was elongated and composed of separate vertebræ, as in the Jurassic Archæopteryx.

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;

a,

Fig. 212.-Toothed Birds (Odontornithes) of the Cretaceous Rocks of America. 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; d, Side view of the same; d, Tooth of Hesperornis 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 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.

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:

6

(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. Portlock.

(8) Recherches sur le Terrain Crétacé Supérieur de l'Angleterre et de l'Irlande.' Barrois.

(9) "Geological Survey of Canada"-"Report of Progress, 1872-73.' (10) Geological Survey of California.' Whitney.

(11) 'Geological Survey of Montana, Idaho, Wyoming, and Utah.' Hayden and Meek.

(12) 'Report on Geology,' &c. (British North American Boundary Commission). G. M. Dawson.

(13) Manual of Geology.' Dana.

(14) Lethæa Rossica. Eichwald.

(15) 'Petrefacta Germaniæ.' Goldfuss.

(16) Fossils of the South Downs.' Mantell.

(17) Medals of Creation.' Mantell.

(18) Mineral Conchology.' Sowerby.

(19) Lethæa Geognostica.' Bronn.

(20) Malacostracous Crustacea of the British Cretaceous Formation' (Palæontographical Society). Bell.

(21) 'Brachiopoda of the Cretaceous Formation' (Palæontographical Society). Davidson.

(22) 'Corals of the Cretaceous Formation' (Palæontographical Society).

Milne-Edwards and Haime.

(23) Supplement to the Fossil Corals' (Palæontographical Society). Martin Duncan.

(24) Echinodermata of the Cretaceous Formation' (Palæontographical Society). Wright.

(25) 'Monograph of the Belemnitidæ' (Palæontographical Society).

(26 of the Trigonia' (Palæontographical Society).

Lycett.

(27) Fossil Cirripedes' (Palæontographical Society). Darwin.

(28) Fossil Mollusca of the Chalk of Britain' (Palæontographical

Society). Sharpe.

(29) 'Entomostraca of the Cretaceous Formation' (Palæontographical Society). Rupert Jones.

(30) 'Monograph of the Fossil Reptiles of the Cretaceous Formation' (Palæontographical Society). Owen.

(31) Manual of Palæontology.' Owen.

(32) Synopsis of Extinct Batrachia and Reptilia.' Cope.

(33) "Structure of the Skull and Limbs in Mosasauroid Reptiles"

American Journ. Sci. and Arts, 1872.' Marsh.

(34) "On Odontornithes American Journ. Sci. and Arts, 1875.'

Marsh.

(35) Ossemens Fossiles.' Cuvier.

(36) Catalogue of Ornithosauria.' Seeley.

(37) Paléontologie Française.' D'Orbigny.

(38) Synopsis des Echinides fossiles.' Desor.

(39) 'Cat. Raisonné des Echinides.' Agassiz and Desor.

(40) "Echinoids"-Decades of the Geol. Survey of Britain.' E. Forbes. (41) Paléontologie Française.' Cotteau.

(42) Versteinerungen der Böhmischen Kreide-formation.' Reuss.

(43) "Cephalopoda, Gasteropoda, Pelecypoda, Brachiopoda, &c., of the Cretaceous Rocks of India "Palæontologica Indica,' ser. i., iii., v., vi., viii. Stoliczka.

(44) "Cretaceous Reptiles of the United States"—"Smithsonian Contributions to Knowledge,' vol. xiv.

Leidy.

(45) Invertebrate Cretaceous, and Tertiary Fossils of the Upper Missouri Country.' 1876. Meek.

CHAPTER XVIII.

THE EOCENE PERIOD.

Before commencing the study of the subdivisions of the Kainozoic series, there are some general considerations to be noted. In the first place, there is in the Old World a complete and entire physical break between the rocks of the Mesozoic and Kainozoic periods. In no instance in Europe are Tertiary strata to be found resting conformably upon any Secondary rock. The Chalk has invariably suffered much. erosion and denudation before the lowest Tertiary strata were deposited upon it. This is shown by the fact that the actually

eroded surface of the Chalk can often be seen; or, failing this, that we can point to the presence of the chalk-flints in the Tertiary strata. This last, of course, affords unquestionable proof that the Chalk must have been subjected to enormous denudation prior to the formation of the Tertiary beds, all the chalk itself having been removed, and nothing left but the flints, while these are all rolled and rounded. In the continent of North America, on the other hand, the lowest Tertiary strata have been shown to graduate downwards conformably with the highest Cretaceous beds, it being a matter of difficulty to draw a precise line of demarcation between the two formations.

In the second place, there is a marked break in the life of the Mesozoic and Kainozoic periods. With the exception of a few Foraminifera, and one Brachiopod (the latter doubtful), no Cretaceous species is known to have survived the Cretaceous period; while several characteristic families, such as the Ammonitida, Belemnitida, and Hippuritida, died out entirely with the close of the Cretaceous rocks. In the Tertiary rocks, on the other hand, not only are all the animals and plants more or less like existing types, but we meet with a constantlyincreasing number of living species as we pass from the bottom of the Kainozoic series to the top. Upon this last fact is founded the modern classification of the Kainozoic rocks, propounded by Sir Charles Lyell.

The absence in strata of Tertiary age of the chambered Cephalopods, the Belemnites, the Hippurites, the Inocerami, and the diversified types of Reptiles which form such conspicuous features in the Cretaceous fauna, render the palæontological break between the Chalk and the Eocene one far too serious to be overlooked. At the same time, it is to be remembered that the evidence afforded by the explorations carried out of late years as to the animal life of the deep sea, renders it certain that the extinction of marine forms of life at the close of the Cretaceous period was far less extensive than had been previously assumed. It is tolerably certain, in fact, that we may look upon some of the inhabitants of the depths of our existing oceans as the direct, if modified, descendants of animals which were in existence when the Chalk was deposited.

It follows from the general want of conformity between the Cretaceous and Tertiary rocks, and still more from the great difference in life, that the Cretaceous and Tertiary periods are separated, in the Old World at any rate, by an enormous lapse of unrepresented time. How long this interval may have been, we have no means of judging exactly, but it very possibly was as long as the whole Kainozoic epoch itself. Some day we