154 Geological Society, 



with the underlying beds than does the mass of the Calcaire grossier, 

 in which the proportion is as 28 to 100. The total number of 

 Molluscs in the Calcaire grossier of the Oise is 651, and in the 

 Bracklesham series of Hampshire 368. 



Above this zone is the series of the Gr^s Moyen in France and 

 Barton clays in England. Owing to the number of Calcaire gros- 

 ser fossils which had been found at Barton, these beds had been 

 considered synchronous with the Calcaire grossier, a view which 

 the author himself had formerly adopted with reserve. Seeing, 

 however, that the Bracklesham series probably represented all the 

 divisions of the Calcaire grossier, and that the distinction between 

 the Bracklesham and Barton series was of equal value to that 

 between the Calcaire grossier and the Sables moyens, the author now 

 correlated the Barton clays with the Sables moyens, as suggested by 

 M. Graves, M. Dumont, Sir Charles Lyell, and M. Hebert. He, 

 however, alluded to the difficulty of doing this upon the evidence of 

 any small number of organic remains, or even of a few species con- 

 sidered characteristic in one area ; and he showed that in the Barton 

 clay itself, although there were many Gres moyen species (63), still 

 there were a greater number of Calcaire grossier species (69). In 

 the same way in the Laekenian system of Belgium, which overlaid 

 the Bruxellian system (the equivalent of the Calcaire grossier), 

 there are forty-five Calcaire grossier and Bracklesham sand species, 

 and only forty-four Barton and Gres moyen species. But Mr. 

 Prestwich showed that, taking the per-centage of species which 

 range from the lower to the higher series, each area offered nearly 

 an equal amount of distinction, as out of 100 species of the lower 

 series there are in England 30, in France 35, and in Belgium 32, 

 which range upwards. 



Mr. Prestwich mentioned that M. Graves had recognized several 

 well-known Barton species, such as the Valuta depauperata, V. athleta, 

 Oliva Branderi, Conus scabriusculuSy &c., in the Sables moyens 

 of the Oise. The total known number of the Sables moyens species 

 is 377, and of the Barton clays 252. 



These series the author proposed to term the " Paris Tertiary 

 Group " (its lower part), as the several members of it were more 

 complete in France than in England, and contained a richer and better- 

 preserved fauna. This Paris group forms the great Nummulitic 

 zone. Hitherto none of these Rhizopoda have been found in the 

 London group. The author concluded with some general observations 

 on the extent of the ancient seas and the position of the dry land, 

 and took occasion to observe on the fact, that, although the several 

 deposits in each country were so rich in organic remains, yet only 

 a small proportion of them had hitherto been identified as com- 

 mon to the several areas. Nevertheless the same genera prevailed, 

 and the relative number of species of each genus was generally 

 tolerably well maintained. He hoped, therefore, that Palaeontolo- 

 gists would, in cases where there was now good reason to believe the 

 strata to be synchronous, inquire into the extent of variation which 

 the same species might undergo in areas where the sea had presented 



