xvi. POT TON BONE BED. 431 



bands of fresh-water shells quietly deposited where they lived, the 

 Potton sands and bone-bed, almost if not wholly heaped in agitated 

 laminse, are devoid of living residents, either of marine or fresh 

 waters. 



Though these beds may not have been quite contemporaneous pro- 

 bably were not they belong to one great system of physical events, 

 connected with changes of level of sea and land. The oolites cease, 

 and the clays connected with them ; with them dies out a large 

 group of marine life, and after them comes in a large and different 

 series of rocks and fossils, constituting the true cretaceous system 

 of Europe. 



FOSSILS OF THE CRETACEOUS SYSTEM. 



These have been as yet only partially collected in the country 

 lying within the limits of Berks, Oxon, and Bucks, but abundantly 

 in the region of Wiltshire, which therefore is included in the 

 catalogue of localities. The lower greensand is exhibited in its 

 most fossiliferous state at Faringdon, where shells and sponges 

 which make up the greater part of the partially-aggregated mass 

 have been collected by Austen, Morris, Sharpe, Wright, and other 

 naturalists. The gault has been frequently explored from Oxford 

 with moderate success at Culham ; the upper greensand and chalk 

 fossils must be quoted from Wiltshire, with hardly any important 

 additions between that rich tract and the equally productive areas 

 round Cambridge and West Norfolk. The palaeontological dif- 

 ferences between the several groups of strata here referred to are 

 considerable, especially if we compare the series of lower greensand 

 fossils with those of the chalk; but so much of analogy runs 

 through the whole as to justify the combination of all into one 

 general catalogue, such as that which follows. It is probable that 

 the defect of localities for the chalk and upper greensand in the 

 region south of Oxford may be remedied by repeated and more 

 fortunate research on the part of the Geological Survey. Some 

 species and localities derived from this source, principally supplied 

 by Mr. Whitaker, are marked by the letters O. G. S. 



