1S95. OCEAN DEPOSITS, ANCIENT AND MODERN. 389 



the Gastropoda with three species, Aporvhais having two only, and the 

 remainder but one each, a remarkable reduction when compared with 

 Blackdown conditions. The area from which these data have been 

 taken is in no sense restricted in its character, embracing, as it does, 

 the Upper Greensand of Warminster, Devizes, Cambridge, and the 

 Isle of Wight. This conclusion is confirmed by various memoirs 

 of the Geological Survey, and Westlake's analysis of Barrois' 

 results, which take into account the Norwich district and the whole 

 of the London Basin. Among Pelecypoda, the foreshadowing of the 

 predominance of Monomyaria is already marked. In the Isle of 

 Wight, Ostvea is represented by 10, Pecicn by 3 species, whereas 

 Trigonia has only 5 ; Lima, Plicatiila, and Spondylus, with 3 species 

 each, have as principal Dimyarian competitors Panopo'a and Cncullcra, 

 also with 3 species each. At Devizes the same facts hold good, Pecten, 

 Ltina, and Inoceranms having each three species, Modiola alone among 

 Dimyaria being represented by an equal number. On reaching the 

 Chalk Marl the distribution becomes exceedingly simple in its 

 character, and in only one county, viz., Sussex, more especially at 

 Hamsey, near Lewes, has any important combination of genera been 

 met with. This combination, however, is one of the highest interest, 

 and is, in fact, startling in its nature, for at this spot, and nowhere 

 else, we find suddenly revealed groupings of Trochus, our only Voltita, 

 Buccinum, Cerithinm, Tnvritella, Turbo, Solarium, Aporrhais, Pholadomya, 

 and Dentalinm, the last three also having been met with together at 

 Dover or Folkestone. It is most remarkable that so many tropical 

 or subtropical forms should be found at this one spot. Lewes is 

 one of the richest neighbourhoods for Cretaceous fossils, and I have 

 suggested that this and other features may have been due to the 

 presence of a great current at this locality. It is also evident from 

 the nature of the fauna that this current must have been a warm one, 

 possibly flowing from the east, and deflected northwards by land 

 areas in the region of our western counties. With this sole exception 

 the rule I have formulated seems to be constant ; the Monomyarian 

 fauna is decidedly predominant, Lima and Ostrea having 6 species 

 each, Pecten 5, and Janira 3. Further details can be gathered from 

 the Survey memoirs, Barrois' work, and the collection in the Natural 

 History Museum. 



From the Belemnitella plena zone (at the base of the Middle 

 Chalk) upwards, the Gastropoda and Dimyaria lose their importance. 

 Inoceramus, Lima, Pecten, Spondylus, Plicatiila, and Avicula are, as is well 

 known, the only genera common in the zones of the Middle Chalk, 

 and by no means rare in those of the Upper Chalk. Other finds 

 should be considered casual, in no way invalidating the conclusion as 

 to the deep-sea nature of the deposit. A large specimen of Plenro- 

 tomaria seen by me at Lewes, which had been obtained from the 

 Marsupites zone, and the Pinna from the Upper Chalk at Bromley, 

 created in my own mind for some time a real difficulty, which was, 



