264 ME. J. W. GEEGOET ON THE 



VII. Affinities of the Fauna. 



The preceding list shows that the Biyozoa inchided in the present paper belong ta 

 three fairly distinct faunas, but a comparison of the three shows that they possess 

 certain features in common. In the first place, each of the three faunas is numerically 

 small, both in species and individuals, in comparison with the wealth of forms that 

 inhabited the contemporary seas of the Mediterranean basin. 



The stunted and dwarfed aspect of the three faunas is apparently due mainly to 

 climatic conditions. As has been pointed out in a recent revision of our Eocene 

 Echinoids i, the British seas of that period were confined to the south by a land barrier 

 which stretched across France and Northern Germany. Hence to the south of this 

 area the Bryozoa flourished under favourable conditions in a tropical and subtropical 

 ocean, while on the other side the seas were open to the chilling influences of the 

 northern ocean. The land barrier was breached in Middle Eocene times, but the 

 conditions were not seriously modified till later : then, with the gradual change to the 

 brackish and freshwater deposits of the Oligocene, the marine Bryozoa cease to be 

 represented in the British Palaeogene. 



The Echinoids of the period belong to the same genera as their contemporaries in the 

 Mediterranean basin, but their generally dwarfed aspect and rareness indicate that they 

 lived under unfavourable conditions. The Bryozoa present exactly the same parallel. 



An effort has been made to explain the paucity of Bryozoa in English deposits of 

 this period as due simply to unfavourable lithological conditions of life and preserva- 

 tion. The prevalence of clay and sharp sand is quoted as unfavourable to the growth 

 of Bryozoa. But this is hardly sufficient. The shelly sands of the Bracklesham, on 

 the contrary, would seem to indicate the conditions that would be most favourable to 

 the existence and preservation of Bryozoa. That the clay shores of the London Clay 

 and Barton are wholly responsible for the rarity of the Bryozoa is not likely to be 

 accepted by any one who has dredged on the great mud-flats off' the Essex coasts, where 

 it is often difficult to procure a shell not encrusted by them. In other districts, such 

 as the Paris basin, Belgium, and North Germany, which were also to the north of this 

 land barrier, and where the lithological characters of the sea-floors were quite different 

 from those of England, the Bryozoa are equally rare and stunted. 



Hence, it is to geographical questions rather than to the lithological conditions of 

 the sea-floor that we must attribute the marked characters of our Palaeogene Bryozoan 

 fauna. 



The singular diversity of the fauna is another feature which supports the view that 

 it is to be regarded as a remnant or an offshoot from one that was much greater and 

 richer. Mr. Waters [Nos. 12 & 13], in his revision of the Oligocene Bryozoa of North 

 Italy, admits 88 species, representing 35 genera. But the British fauna contains only 



' Gregory, Proc. Geol. Assoc, xii. 1891, pp. 51, 52. 



