(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 _Ammonitidoe, Belemnitidoe_, and _Hippuritidoe_, 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 constantly-increasing 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 Sil Charles Lyell.