92 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



are so singularly rich in organic remains, that they there afford on 

 all points the best types to which to refer the English and Belgian 

 beds. The divisions I have adopted are those of M. D'Archiac, mo- 

 dified in parts by those of M. Chas. D'Orbigny and of M. Graves, 

 the latter of vv^hose valuable work on the Beauvais and Chaumont 

 district affords the most complete data for the comparison of the 

 French with the English Eocene faunas. 



In comparing the fauna of the several deposits I have confined 

 myself as before almost entirely to the shells alone, as these organic 

 remains are not only far more numerous than any others, but have 

 been much more fully worked out. Still so much remains to be 

 done even with the shells, that M. Graves and many other French 

 geologists have abstained from defining the exact number of species 

 common to the several deposits. Nevertheless I believe that the 

 lists they give are sufficiently complete to guide us at all events in a 

 preliminary inquiry and to show the direction which the argument 

 should take. This is the more essential, as with a perfect identity in 

 many common fossils, there are nevertheless important differences 

 in the totality of the faunas of the French and English formations, 

 whilst in lithological structure and thickness they also present but 

 few points of resemblance. 



The Bracklesham Sands are about 500 feet thick in the Isle of 

 Wight, whereas the Calcaire grossier does not exceed, taking each 

 division of it at its fullest development, 140 to 150 feet ; but as the 

 several divisions are not all in full force in any one place, the actual 

 average thickness rarely exceeds 100 feet. The French formation is 

 however much richer than the English in organic remains. M. 

 Graves enumerates a total of 824 species, whereas at Bracklesham 

 there are only 451 known species. Amongst the latter there are 368 

 molluscs, and in the former 651. Of this large number only 144 

 species have been at present determined to be common to the two 

 countries, and yet there can be very little doubt that these for- 

 mations are perfectly synchronous ; the difference, I believe, arising 

 both from the different geographical conditions which commenced to 

 obtain at the period of the London Clay, and also to the differences 

 of the sea-beds of the two regions. The general list of the Brack- 

 lesham fossils I have given in a previous paper*. I now annex a 

 list of all the Bracklesham shells identified with French species, with 

 columns showing the extent of their range either in the Calcaire 

 grossier or in higher or lower bedsf- Although the several divisions 

 of the Calcaire grossier fossils pass one into another, still the species 

 are differently distributed, and there is a marked distinction as a 

 group between the fossils of the lower and upper zones : they are 

 far less numerous in the latter, and some few are peculiar to each. 

 In the following Table I have therefore found it convenient to take 

 the two lower and the two upper divisions separately. 



* Quart. Journ. Geol. Soc. vol. x. p. 450. 



t Notwithstanding every care, I feel satisfied that both this and the Barton list 

 (p. 118) yet require considerable correction. Of their essential truth I, hovrever, 

 feel more convinced. 



