species of the London Clay, which had disappeared during the period of the intervening Bracklesham Sands, reappeared with the reoccurrence of argillaceous strata. One of the most remarkable cases, however, is that of the Argile de Boom, which forms the very top of the Eocene series of Belgium, — the Oligocene of German geologists. This deposit is so like the London Clay in lithological character that it would be almost impossible to distinguish them, while the shells (especially the several species of Fusus, Pleurotoma, and Natica) so closely resemble those of the London Clay, from which it is separated by the four or five divisions of the Upper Eocene, that they might easily be mistaken for London-Clay fossils. The exceptional appearances of Colonies, whether in the older or newer rocks, are, no doubt, mainly due to the recurrence at certain intervals of similar lithological, thermal, and bathymetrical conditions.
During the Middle Tertiary or Miocene period, it would seem that a different distribution of land and water prevailed. The Miocene beds of Skye and of Greenland, with their remarkable floras, indicate land and fresh-water conditions, while at the same time the Miocene marine beds of Prance and Germany are rich in subtropical forms of Mollusca. Assuming part of the area which now constitutes the Northern Atlantic area to have been then dry land, the migration southwards of arctic species of Mollusca would have been for a time interrupted.
Approaching nearer to our own times, we have Pliocene beds in Iceland, Quaternary deposits in Spitzbergen and on the western flanks of the Scandinavian peninsula, while in this country Glacial or Preglacial beds range to the height of from 1000 to 1400 feet above the sea-level. There is reason, therefore, to believe that the bed of the North Atlantic may have been from 1500 to 1600 feet or more deeper during the Pliocene and Glacial period than it now is. If northern submarine currents are now checked, as Prof. Wyville Thomson supposes, by the shallower seas between Scotland and Greenland, such an addition to its depth as these emerged portions indicate would materially have affected those conditions, and have allowed of a freer passage of the north-polar waters, and consequently of a freer dispersion of its fauna to the abysses of the mid-Atlantic, where, in fact, so large a number of them are now found to exist. This more open communication gave rise, I conceive, to that great migration of northern Mollusca which are now found fossil in Italy and Sicily, and some of which still survive in the Mediterranean and mid-Atlantic.
