322 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



producing 76 species in a thickness of less than 6 feet ; while the clay- 

 next above, 60 feet thick, gives but 6 new species ; the " Crackers'* 

 group again, over the clay, between 80 and 90 feet, giving 48 new 

 species, besides those brought up from the lowest part of the section. 



The comparative range of these species is also deserving of notice : 

 24 which had just appeared in Nos. 1 and 2 go no farther ; 5 dis- 

 appear in the clay No. 2 ; and 45 more go no higher than the Crack- 

 ers, Nos. 4 to 10. So that the groups I. and III. are not only the 

 most productive of new species, but are marked also by the sudden 

 extinction of about 70 of those which had originated within them. 



The most obvious inference perhaps from the first view of this great 

 disparity might be, that the lower part of the section ought to be sepa- 

 rated from the upper strata, and form a division apart ; and if relative 

 numbers alone were to be taken into the account, and it were found 

 that the disproportion existing here was of general occurrence in di- 

 stant situations, such a separation might be expedient ; but the uni- 

 versality of such a state of things has certainly not yet been proved ; 

 and the question of detaching these lower strata from the upper series, 

 with which they are conformable in position, and bear every appear- 

 ance of having been continuously deposited, is not simply numerical, 

 but is intimately connected with the zoological relations of the animals 

 whose remains are found in the strata. 



The explanation therefore of this extraordinary distribution of the 

 fossils, given by Professor E. Forbes in an early number of this Jour- 

 nal *, seems to be well-founded ; and it is supported by reference to 

 the habits of marine animals, and their mode of distribution at 

 the bottom of actually existing seas. According to this view, the 

 Perna and the Crackers group — deposited immediately after the de- 

 pression of the Wealden shore into the sea of that period, were pro- 

 duced under circumstances which were peculiar and exceptional. The 

 sudden disappearance or extinction of many of the species which we 

 find them to have contained was not a change of inhabitants, but an 

 absolute depopulation to a certain extent, for no successors or repre- 

 sentatives of the extinguished species have been discovered. But a 

 residuary portion of the occupants of the sea was continued, receiving 

 but few additions, and rapidly thinning out, till at length, on the 

 appearance of the gault, the whole was changed ; so that, in a general 

 view, one system only of organic remains pervades this series, the same 

 species recurring or reappearing wherever similar conditions prevail ; 

 while the occasional presence of other and additional animals seems 

 to have arisen from incidental and local circumstances more or less 

 favourable to their wants and mode of existence. 



Thus, while a general abundance of fossils almost everywhere di- 

 stinguishes the lower part of the sub cretaceous S3^stem, variety in the 

 distribution of species, and not precise identity, in different localities, 



* Journal of the Geological Society of London, vol. i. p. 192, &c. In refemng 

 to this puhlication, I ought to add, that my impressions upon this subject (which 

 I hope have been correct) were derived in part, or principally, from conversations 

 with the author, and from the discussions at the meetings of the Geological 

 Society. 



