The fauna of the Mediterranean naturally presents a considerable difference from that of the Atlantic, especially from that portion which inhabits the greater depths of the latter. There is an absence of the numerous recent arctic forms which follow the cold currents of the Atlantic, although there are many northern forms of Quaternary and Pliocene age, which seem to have been introduced into the Mediterranean area at a period when the communication between the two seas may have been more open — an inference made by several observers both on natural-history and on geological grounds. Newer Tertiary strata extend, in fact, a great part of the way across from the Bay of Biscay to the Mediterranean, and the watershed between the two seas is not higher than about 600 feet above their levels. At one point on this line, and at an elevation of 560 feet above the Mediterranean, M. Virlet d'Aoust many years since discovered, in a fossil state, the Ostrea hippopus and Murex trunculus, species still living in that sea.
From these considerations the question arises whether the deep sea in which the Chalk, with its more tropical genera, was deposited, may not also have been a sea shut out from direct communication with Arctic seas. The Old and New continents have a north and south extension, with intervening oceans in the same direction ; but the distribution of land and water must have been very different during the Cretaceous period. Beds of this age stretch from England through France, Germany, Poland and Southern Russia to Persia and India, and they also traverse the southern portions of the North- American continent. Throughout much of Europe and parts of Asia the Chalk has the common character that it possesses in England, and which has led it to be likened to the Atlantic deep-sea mud. On the other hand, there is no Chalk north of Denmark, in North Russia or Siberia, or in Arctic America. If the direction of the deep Chalk-ocean followed this east and west belt across the present continents, then we must look for dry land on the confines of that ocean ; and it is probable that the latter may have been, to the north, in the direction between Greenland and Scotland and Scandinavia, where the present ocean is some hundreds of fathoms shallower than further south. We know that towards the end of the Cretaceous period, a change took place in the fauna, arising apparently from the shallowing of the sea that preceded the deposition of the Maestricht beds, as well as of the Calcaire pisolitique of Laversine and Mont Aime. Many of the great Cephalopods disappeared, and reptiles increased in
