ANNIVEK8AKY ADBBESS OF THE PEESIDENT. Ixvii 



such as here occurs ? Can it be that such a group of generic forms, 

 allied to and closely resembling those found in the same zoological pro- 

 vince at the present day, had a yet older existence in more northern 

 provinces — that generic forms of temperate regions have travelled 

 ^om the north, and have been gradually spread further south, 

 giving, when they encroached upon the more southern forms, a more 

 recent aspect to the faimas of such various geological periods than 

 prevailed in those of the same localities when changes in the distri- 

 bution of land and water brought back for a time the southern forms 

 which had been temporarily displaced ? " 



That much of the difference between the fauna of the Chalk and 

 the Lower Tertiaries must be due to the elevation of the old Chalk 

 ocean-bed (by which the deep-sea life was exterminated and a shal- 

 lower-water fauna introduced) is now evident from the recent deep- 

 sea dredgings. Suppose, for instance, a portion of the present bed 

 of the Atlantic were raised to the level of the sea-bed of the present 

 English channel, whereby the depth of water would be reduced 

 from 12,000 or 15,000 to 100 or 600 feet. The deep-sea fauna 

 would be destroyed, and the fauna and sandy beds of the EngKsh 

 coast would succeed it ; and when these were raised, we should 

 have sand and gravelly beds containing a shallow-water fauna 

 overlying calcareous beds with a deep-sea fauna, and there would 

 be but very few, if any, species common to the two deposits. 



As old coast-lines and the oceanic currents changed during the 

 Tertiary periods, we may suppose corresponding changes in the fauna 

 of the littoral and laminarian zones, while the deeper-sea fauna (which 

 was not subject to these changes of conditions) may have had a much 

 longer and more permanent existence. Together with the recurring 

 bathymetrical conditions, the lithological character of the sea-bed 

 further influenced the vitality and persistence of species. The Mol- 

 lusca of the Calcaire grassier of the Paris basin are, according to M. 

 Deshayes, essentially southern in their character and relations. This 

 formation is separated from the Chalk by the London Clay or its equi- 

 valents, and the Woolwich series and Thanet Sands, with the fauna of 

 which it has few species in common, whilst, as I have before men- 

 tioned, the species of the Lower Eocene beds have a more northern 

 facies. It is not, however, long since MM. Cornet and Briart found 

 under the equivalents of all these English series in Belgium a friable 

 calcareous bed fuU of fossils, not like those of the overlying Lower 

 Eocene, but resembling, and in many cases identical with, those of 

 the more recent Calcaire grassier. Again, in the Barton Clay, many 



