1^47*] PRESTWICH ON THE LONDON CLAY. 375 



the shallower waters gradually disappear. At the end of this period 

 the latter again prevail to the near exclusion of the former. 



From these more varied conditions, and from the operation of more 

 numerous zones of depth, adapted to the existence of more numerous 

 classes of Testacea, results prohahly the more diversified and abun- 

 dant fauna of Hampstead, Highgate and Sheppey. 



Comparison of the London Clay with the French Tertiaries. 

 In my paper on the Tertiary Geology of the Isle of Wight, I ex- 

 pressed an opinion that the Bracklesham Bay beds were of the age 

 of the lower Calcaire grossier and the Glauconie grossiere. Since 

 then I have had the opportunity of examining the large and valuable 

 collection from Bracklesham of Mr. Edwards, and am confirmed in 

 this view of their synchronism. The number of French species found 

 at Bracklesham amounts to about 140 out of the total number of 193 

 described species. Of these 140 about 120 occur in the lower Cal- 

 caire grossier and the Glauconie grossiere ; about 40 descend to the 

 Lits Coquilliers ; and still fewer are peculiar to the upper Calcaire 

 grossier and bed above ; thus showing a far more intimate connection 

 with the first-named divisions than with any of the others*. 



There are a few anomalies, but they are overbalanced by the 

 stronger coincidences. Thus, among the few characteristic fossils of 

 the Lits Coquilliers in France, we find foremost the Bifrontia Lau- 

 dunensis and Nummulites planulatus (elegans. Sow.). Both these 

 species occur in considerable abundance at Bracklesham, but associ- 

 ated with them we have a large proportion of the characteristic spe- 

 cies of the lower Calcaire grossier and Glauconie grossiere, including 

 with many others, 



Cerithium giganteumf, Lam. Pecten squamula, Lam. 



Conus deperditus, Brug. Pleurotoma granulata, Desk. 



Fusus No8s, Lam. Voluta spinosa, Lam. 



Lucina gigantea, Desh. cithara, Lam. 



concentrica, Lam. Venericardia planicosta, Lam. 



Melania costellata, Lam. acuticosta, Desh. 



Of the 209 species of Testacea found at Barton, about 100 are 

 identical with species occurring in France, of which 30 are found 

 in the Lits Coquilliers, about 60 in the Calcaire grossier and Glau- 

 conie grossiere (more particularly the lower Calcaire grossier), and 

 probably 20 in the overlying beds. The number of French analogues 

 is here much less than in the Bracklesham series, and with a larger 

 proportion of species of the Lits Coquilliers. This is peculiar, as 

 from superposition there can, I think, be little doubt that the Barton 

 series are higher than the Bracklesham Bay series. Before however 



* At Chauraont, between Rouen and Beauvais, the Glauconie grossiere literally 

 abounds with fine specimens of the Venericardia planicosta. It is the same spe- 

 cies which constitutes so typical and common a form at Bracklesham, and in the 

 beds of the same age at White-Cliff Bay and Southampton. From lithological 

 character and superposition, it is more particularly with these beds that I should 

 associate the Bracklesham sands and clays. I have not however visited Chaumont, 

 nor have I been able to meet with any full account of its sections and fossils. 



t A French author, in speaking of the lower Calcaire grossier, refers to it as 

 " I'epoque du Cerithium giganteum, cette coquille-reine qui est devenue une date 

 geologique." This however may be doubtful. 



